Prepare yourselves, ladies (and gents), this is a bit of a long one.

I've (25) been in a relationship for about 10 months with my SO (29). During the early stages of our relationship, around 3 months in, I caught my boyfriend

1 exchanging sexual pictures with an old hookup and

2 cheating on me during a party (yes, I literally walked in on this scene)

In spite of these two instances, we decided to stay together. He has done his best to show me he's trustworthy, and I have done the same for him. (I've never cheated on him but I still think this is important.) He is no longer in contact with the two girls from the party, and he showed me messages he sent to them saying they would never be allowed back at his house, etc. He also showed me a message he sent to the girl he had been exchanging pictures with, saying that he would not be doing that anymore. However, he is still occasionally in contact with her. I often find the fact that he remains in contact with her stressful, and it makes me incredibly anxious and insecure. But I also don't feel it's right or my place to be telling him who he can and cannot communicate with. Not only do I never want to be the type of person who forces her partner to stop talking to certain people, but I am also confident that in doing so I would only make it more tempting to be in contact with that person.

I have expressed to him a handful of times that this girl makes me unconfortable, and he always reassures me that there is nothing between them, and in any case she is no longer single, but I rarely actually feel reassured. I know that building back trust is difficult and takes a long time, and surely seven months isn't enough to repair the damage of what happened in the past, but the problem is that I don't feel like things have been getting better lately.

I am almost 100% certain that he is no longer engaging in that kind of behaviour with this girl (I can't say it with 100% certainty because I don't want to look through his phone. I respect his privacy in spite of everything) but I still feel incredibly insecure, and like his second choice. Logically I know this isn't true, as he has told me many times that he would not be happy being in a relationship with her, and that he doesn't want to be with her or anyone else that isn't me, but in low moments I feel myself becoming more and more obsessed with the idea that he still wants to be with her and not me. My only "proof" of this? He reacts to her pictures on facebook. I know, I know -- it seems ridiculous. But I have no idea how to stop myself from obsessing over this, and I know that if I continue down this path of insecurity and suspicion, it won't be anything he does that will end the relationship -- it will be me.

In all other regards, our relationship is very solid, and in spite of 10 months being a relatively short amount of time, we are both interested in this being an extremely long-term (aka "for life" if possible) relationship, with children etc.

If anyone else who decided to stay in a relationship after cheating has advice for me -- or even people who haven't been in this particular situation -- I would be incredibly grateful.