Slept with my best friends boyfriend. Watch
I can't believe what I have done. I feel so guilty. He won't say anything, and obviously I won't. But I don't know what to do now. I don't even want to see him ever again. I'm mortified. I am a horrible person. What do I do?
Explain that you were drunk etc but also acknowledge to her that you know that it in no way excuses your actions.
You have to let her decide what to do after she knows.
Your choice, now you deal with any consequences.
I'll say the quicker you tell your friend this happened, that there is some greater chance of things being resolved positively. At least, more positively than if you leave things to linger. Isn't going to be easy, as you may have jeopardised your friendship badly.
We all make mistakes. Esp. stupid ones when we are drunk. but you need to come clean to your friend- your BEST friend. If she stops being your friend, atleast you did the right thing and came clean. Otherwise your friendship from now on would be a lie. I know its hard.. But you never know, there is a slight chance she might forgive you since you came clean. I wouldn't wait too long to do it though.
Also, don't drink that much again. I don't know why people try to claim diminished responsibility because of drink - nobody forced it down your neck.
What you do next will depend on what kind of person you want to be in life. There is no right or wrong. I would say the closest thing to 'right' would be to tell your friend immediately, without explanation but no excuses, and take the consequences. However, I know it's not that simple in reality.
It'll be hard for her but she's currently got a boyfriend who is willing to cheat on her and a best friend who is willing to sleep with her boyfriend and she deserves to know so that she can cut contact if she wants to.
If you're really her best friend and you genuinely do care about her (questionable by your actions so far...), you owe it to her to tell her the truth.