Why? Why things like this happen?

I had to go away for a little over a week for work last week. Her cousin passed away on Wednesday, the funeral was on Saturday. I never really met this cousin and they had a falling out before she and I were together but they used to be close like sisters and were neighbors growing up living on the same street. They saw each other when they couldn't avoid it but generally avoided each other.

On Sunday morning I get a text saying I love you so much, I'm sorry but we need to talk when you get home. I started calling like crazy because that's the worst thing possible you can do IMO sending a message like that. Unfortunately her phone stopped ringing so she must have switched it off. I was worried and panicking most of Sunday and yes my mind jumped to cheating.

So I get home yesterday and she's there and tries to hug me but I pretty much just tell her to say what she needs to say because I was angry she just messaged that to me then went dark. Her cousin died and she was devastated despite their falling out. At the wake on Friday she ends up meeting a family friend and drinking a bit. Then on Saturday after the funeral they end up spending more time together in a group then alone before she had sex with him several times on Saturday night. When she woke up on Sunday it hit her just what she'd done and she freaked out.

According to her she was devastated, not thinking clearly and just needed something to take her mind off of what happened. She was apparently in a really bad way and wasn't thinking about me or us just then. She's been nothing but apologetic to the point of near groveling since yesterday.

But Idk, I'm devastated, I feel sad because I knew she was always kind of torn up about her falling out with her cousin but this doesn't excuse her hopping into bed to grief fuck away her emotional pain. I mean jesus christ we've been together 6 and a half years, our wedding is in October. And I couldn't even sleep yesterday or last night because I can't stop getting enraged thinking about her being with another man regardless of the reason.

I looked at her this morning and felt nothing except intense sadness and disgust. She tried hugging me and I just left and went for a drive. I took the day off today and I'm regretting that because I'm just at home now with this shit playing through my mind.

TL;DR Fiance of 6 1/2 years grief cheated on me with another guy while I was working away after her cousin's funeral

UPDATE/EDIT: I did not read through every single comment yesterday. However the ones I did read were enough for me to do some thinking last night and realize that I likely won't ever be able to get past this. It would take a truly herculean effort for me to forgive her and move past this and there is every chance in the world that I might just believe I meant it when I forgave her but the pain from her cheating would remain and then reopen like an old wound at a later date.

There's also the whole trust issue thing - I travel for work quite a bit, at least once every two months and at this point I don't know that I'll be able to travel with the peace of mind I used to have before this weekend. I was cheated on in the past and I feel like anytime I get on a plane now I'm going to be worried she's off fucking some other man. I realize this weekend was a complete one off, I know some people may not believe that or agree with it but I know her better than most.

I really do believe her when she said it was a terrible mistake and she wouldn't do it again. I still love her immensely. But I wasn't worth being faithful to after 6 and a half years together so she's not worth giving a second chance. It would be a massive and foolish risk on my part. All she needs to do is be forgiven. I'm the one who would have to work through the pain of being cheated on, the task of trying to forgive her and making sure that I mean it and ultimately going through the extremely painful task of learning to trust her again. So I'm done, as much as that hurts to even begin thinking about and despite the fact that I still love her.