I ended up sleeping with my best friend’s ex and now she hates me...
A couple of weeks before I did it I asked her hypothetically if it would be okay and she said no, however, when I [slept with him] she and I were having severe problems and I saw her ex as more my friend than as her ex. He was very close to me at the time and I had feelings for him and I acted on them. I regret what I did and now I have to choose between him and her and I’m not sure that I can do that... what do I do?
You act as though you haven’t already made a choice.
Here’s what I wish you had said to me:
I asked my best friend if I could have sex with her ex. She said no. I fucked him anyway. I now want to continue hooking up with him and avoid any negative consequences that might affect my relationship with her. Can you help me?
To which I’d say, NO. I CAN’T.
Actions have consequences. Yours should include both being dumped by the guy and by your friend. They both deserve better.