I basically cheated on my gf with my old best friend’s current best friend

I’m probably going to break up with my gf and not give her proper reasoning because, get this, I’m her first normal boyfriend! She’s a virgin and I’m somewhat experienced and I can bet this was a fear of hers…

The worst thing is I really like her I was high(not an excuse) and things rolled to that direction

I feel horrible, probably cause I am a bit.