<human>: Give me the gist of the text in a nutshell

We had a FWB relation going for a month, but we were best friends for about a year.

 It's obvious he has feelings for me (he told me) and I absolutely have feelings for him as well (I told him, but also said I wanted to "take it slow" due to a recent breakup. I told him I did not want to be exclusive.) For me, this meant not becoming too attached to each other too quickly. It ended up happening regardless, we have such an amazing friendship to ground us and falling for him was natural. I was happy about getting closer to him, but scared due to past relationships..

I fucked up. I slept with my ex. I caved in. I am a stupid, weak, horrible person for doing this. I told my FWB about it as soon as I could. He said he would call me back but hasn't yet. 

How could I possibly apologize for doing something so blatantly stupid? Who the fuck would forgive someone for fucking up like that? He probably thinks my feelings weren't real now. He probably thinks I like my ex more than him. I **dont**, he was abusive! And yet stupidly I still went back!

When I was with my ex I kept thinking about how FWB was so much better in every single way. Afterward, he was all I could think about for hours on end. I completely fucked up. How can I possibly attempt to fix this? I'm going to tell him how I feel when he calls, but to be honest I want to hear what he has to say more than anything.
<bot>: I'm such a fuck up. I fucked an abusive ex when I should have been focusing on my best friend, my lover, my inspiration.