My husband cheated with my best friend

We were on a vacation together with my best friend and her boyfriend, they got drunk and she went to him in the shower. They touched each other before I came nearby. I was naive and believed him when he said she was in the bathroom just to get something and she didn't see him in the shower at all. The next day they did something infinitely worse while I was already in bed. Not full cheating, but enough to vomit.

On the way back from vacation, they kept looking into each other's eyes in the rearview mirror. This was the day of our 8 year anniversary. Then they started texting a lot, secretly met for a walk in the forest where they kissed, and sent each other nudes. When they decided they cannot be together, they confessed to us as their partners, however my husband didn't tell me everything, he left out the nudes and I found later through BF's boyfriend. It has been 2 months and his last message to her is "I love you".

What hurts more is probably her betrayal, but what he did is also completely wrong. He has already hurt me once very much 3 years ago (it had something to do with her) and I gave him a second change. But I haven't been perfect either, I kissed my colleague on a party when I was drunk so I don't want to be too hypocritical, but obviously it doesn't make what he did ok.

We have had a rough marriage due external circumstances, husband's father passed away a year ago after a couple months of illness. So he was carrying this huge wound without trying to truly heal it. He fully embraces that he made a mistake, is very remorseful and I believe him when he says he won't do it again. He loves me "more than himself" and is fully committed to fixing the marriage. He feels nothing special for her at this point (she however is still obsessed with him). I love him very much but i'm so deeply angry. He keeps frustrating me with the littlest things. And to be honest, I just don't think I have the energy to rebuild this relationship again. We always had a beautiful relationship but I had been feeling a little trapped in it, that's why I escalated with the colleague before everything happened.

I needed space, so I lived alone for a week. I missed him very much and thought that I do want to work on our marriage. But then in a moment of drunkenness I invited the colleague to the cinema and it quickly escalated and we're sort of together? I'm very happy about it, but at the same time my soul is being ripped into two pieces. I feel very guilty for making my husband absolutely miserable. I know I'm not responsible for his feelings and I shouldn't stay just so he's not sad, but it feels like it's all my fault. It's my decision and I have to take the consequences, but the consequences are so heavy. I can't even imagine a divorce, there is too much shared history, family, possesions, plans and dreams. I'm very confused about my feelings and I don't know what I want.

I'd appreciate external views of people who were/are in a similar situation. I keep being pulled in both directions (staying and not staying) and I don't even emotionally process anything at this point. I'm worried I'll throw away my marriage for nothing and end up alone, which terrifies me. Being stuck in a dysfunctional marriage is equally as terrifying tho. What do you guys think?