I cheated on my partner, now I feel confused and disappointed in myself
Is that a joke? Has it ever happened to you before?
No, but in a different situation it would have been fine.
You should feel guilty for cheating, someone else could have got hurt. Nobody likes people who cheat to get ahread.
My girlfirend won't know but what about the other girl?
I believe it's best to stop this relationship and let all parties know. They don't deserve to be hurt in the back like this, and taking responsibility for what you've done will show a lot of growth to them. They might be mad at first, but relieved you came clean.