Before I start this thread, where I live there are currently no covid restrictions so I'm not breaking any rules. I'm feeling absolutely awful so please be nice 
So my best friend of 15 years slept with a boy last year after a night out. We both know the boy and after sleeping together she became a bit obsessed with him but he wasn't interested in her and just saw it as a one night stand.
Last night I went to a small gathering at a friends house and this boy was there. After getting far too drunk me and him ended up sleeping together. Afterwards we both agreed not to say anything and to just forget about it.
But now I am feeling absolutely awful, I honestly can't stop thinking about it and I feel like the worst person in the world. I know my friend would be absolutely devastated if she knew about this which is just making me feel so so bad. I feel as though I can't function because the guilt is eating me up I really really regret this
I was just looking for advice and to see if anyone had any similar experiences? I love my friend to pieces and actually can't believe that I even did this I feel ashamed of myself.