I just side swiped a car and didn't leave a note
Why wouldn't you leave a note?
I don't know, I forgot to write the note.
Well, sometimes we forget to do things and that's fine.  However, in the future it's important that you take responsibility for damaging someone's property.  It's only fair!
I know I fucked up
Hopefully in the future you remember this feeling.   That way you won't forget again.  Don't beat yourself up too much though!