(sorry for long post, TL;DR is at bottom)

I’m not going to waste time trying to convince you all that he’s a great boyfriend, because he is, but I know that isn’t the point of posting here. Just know that I love him and I really don’t want to leave him due to him being a really great match for me in every other way, never doing anything to break my trust besides this, and him trying really hard to make me happy. I genuinely love him for who he is. We’ve been together a year and moved in with each other a month ago. If any more details are needed for sound advice, I will reply to comments.

Basically, he has been friends with the same group of people since they were young teenagers, the group ranging from 13-16 year olds back then. Out of the group there are three female friends. He was honest with me that he’d kissed another friend when they were younger, and nothing ever came of it, they’re just friends now. I want to emphasize I was and still am totally cool with this, they worked out on their own that they weren’t good together and it’s no big deal to me. They hang out alone sometimes and it literally does not bother me at all.

However, with this particular girl he lied. Apparently they kissed when they were 14/17 (she kissed him, he insists now it wasn’t just a peck and he told her the next day nothing could come of it because she was too young and he also didn’t want to put their friendship on the line if things turned sour). Not a big deal if he had just told me, but the problem is he didn’t. Right before 6 months into our relationship I found out from a message while I was looking for a message containing important info (I used the search bar and it took me back to a message he had sent before our relationship that said “I’m in love with (best friend)... and had the same word in it.) (Yes, I did have his permission to use his phone and the search bar.) The messages were from April 2017, long before we were together.

I confronted him since he’d said before this that this friend and him never made any physical contact beyond a hug. The messages also detailed deep feelings on his side, saying that he loved her deeply, had been hiding it for years, etc. You can see why I feel like an insecure mess.

When I confronted I asked for the truth, told him not to spare my feelings. He said (and has insisted every time we’ve talked about it since that night):

-He was having a mental breakdown and at the time (which I believe, he had horrible trauma around this time), the group was pushing him away due to some other drama and friend was the only person who was nice to him and spent time with him outside of his family, so he was confused; he says now that he has mental clarity he realizes he never loved her, it was infatuation that went on and off for three years when they were older.

-She never knew anything about it, he never told her, the only feelings that were ever discussed between them was the conversation after she kissed him and he told her they couldn’t be together.

-He knows it was wrong because she had a long term boyfriend (whom she’s still dating) and she was too young for him to date at the time, so he didn’t bother to tell her his feelings and says that after a while they went away on their own and did not return.

-He has given me several reasons why he doesn’t want to date her then or now, including: she won’t get a job, she won’t travel anywhere longer than ten minutes outside of their town (him and I are all over the place on our days off from work, we will drive for hours and go explore new cities/places which he really enjoys), her mother is pretty awful, they don’t have the same views on politics, they’d argue too much if they had anything more than friendship because they don’t get along well on a lot of serious subjects so they just avoid those topics, etc.

-He insists he is no longer attracted to her and that he loves only me, had no feelings for her when we started dating, and has never had any feelings for her throughout our relationship.

-He says he never told me because he’s embarrassed that the feelings/kiss ever happened and wants to forget about it, and also because he didn’t want me to feel insecure around her when she really is just his friend. The kiss with the other girl was a one-off thing which is why he told me, since these feelings lasted a while he didn’t want me to feel bad about it when he says I have no reason to (not about the lying, but no reason to feel bad/insecure about being around her). He says he realizes now that this was the wrong choice and has apologized about it several times, and says he wishes he would’ve been honest about it from the start.

I asked him to put some boundaries in place immediately after I found out for my own comfort, such as not getting blackout drunk around her if I’m not there, not going to her house alone (if the group’s there I don’t care), not touching more than a hello/goodbye hug which he agreed to and still does to this day. I told him myself I didn’t want him to stop being friends with her entirely, both because this is an ‘us’ problem - it’s not her fault that he lied to me, and cutting her out won’t fix that - and also because honestly, when I remove the feelings I have over my boyfriend’s lying, I like her as a person a lot.

He’s offered to let me see his messages, he talks to me about it when I get upset and seems genuinely remorseful that he hurt me. He has been texting her and hanging out with her less since it happened as well because he sees how much it hurt me then. We only see her together about once a month now and we party at her house with the group once or twice a month. This is cut back a lot, we used to go over there about twice a week before the issue happened.

I genuinely appreciate what he’s done to correct his actions and I think he is trustworthy now, objectively. I still trust him on certain things; I know he won't cheat on me, I know he won't go out and even say anything disrespectful about me or our relationship, I know he will fight for us and I trust him to listen and act when there's a problem. When we have an issue, he fixes it permanently, not just for a few weeks like past partners. I want so desperately to trust him unconditionally like I did before, but my brain won’t let me believe some of what he tells me anymore. I know he’s doing a lot to apologize and fix the behavior which I appreciate. Tbh he gave me his passwords and gave me full permission to fine comb anything I wanted, and I have since then a few times, and found absolutely nothing. He never chatted with her in an inappropriate way before or during our relationship (I asked to see the messages immediately upon finding out and he just handed me his phone, no questions asked). My boyfriend is very loyal, I know he isn’t cheating because we work the same shifts and share a car at this point. I know he loves me and only me. I want so badly to trust him and he’s trying so hard to hold up his half, he does whatever I ask, but I don’t know what I can do for myself or ask more of him to heal on my end.

I know all of this also happened a long time ago and it's been good since then; I just have some days that are harder than others. Sometimes I just think about it for an afternoon and it really works me up, at which point I go to him crying and he calmly restates the previous bullet points while hugging me/comforting me. It happens less often now as well, when I first found out I obsessed over it and was angry on and off for a couple weeks, which he comforted me through patiently. Now it only happens maybe once every few months. I occasionally feel insecure around her a little more often than that, maybe once a month or so, and we've gotten to a point now where I can talk about that without freaking out/sobbing. There is still an irrational part of me that's very worried he's holding onto feelings since he never told her, which is where I think a majority of our issues stem from - if I could just trust his word that he doesn't have feelings for her, we probably wouldn't be going through these problems. I'm aware of that and I'm really trying to work on it, but because I'm still young and don't know myself very well, I don't entirely know how to help myself in this type of situation (I know I can dump him and feel better, but again, I'd really like to work through it; even if we don't marry, I feel this relationship is very beneficial, loving, and is teaching both of us a lot).

TL;DR: Boyfriend is best friends with girl. Girl and him kissed years ago and he had feelings for her up to a year before we dated, so none of it is infidelity in the slightest. However, he lied about it to me (told me they’d never had physical contact beyond a hug and he never had feelings for her) to avoid me feeling insecure about her and also because he’s embarrassed it ever happened. He’s held up every boundary and action I’ve asked him to take and cut back contact with her on his own without me asking, he’s really trying to earn my trust back and is remorseful for what happened. I still feel like an insecure mess about her and can’t trust what he says, and all I want is to gain that trust back in him. He’s doing everything right now but idk how to heal, advice would be very appreciated.