I fantasized about my best friend's girlfriend
What did you fantasize about your friend's girlfriend?
I fucked her by myself several times and then came
You can imagine what you want, but that's still a little gross and taboo to think about someone else's girlfriend like that. While it's good you kept these thoughts to yourself, you must make sure to not actually act on such things and destroy their relationship and your friendship.
It's not that bad to think about someone you know, besides she's hot.
It can be misconstrued as being provocative to fantasize about your best friend's girlfriend. You should try to watch some porn if you need to fantasize about a girl. Distance yourself from this person and see if you can free your mind of this fantasy.