My response has 2 parts. this is some real talk...from a dude thats been in your shoes...(except the baby momma part)
1. welcome to the world of the scumbag
this is only the beginning breh...a few months going by is all you need for your brain to rationalize what you got away with. you feel like dirt now, but that feeling will fade in a few months. and the next time some strange throws it at you, your brain will tell you to do it..and you will rationalize it by knowing you got away with the first one. this is a mental thing that you have no control over. its over
2. your girl will find out sooner or later because not only did you fukk an ex...you fukked the one ex that you will be tied to for the rest of your life
sorry to laugh breh but you fukked up royally. you better tell your girl because SHE WILL FIND OUT EVENTUALLY....and this will hurt her.
plus your now offically a cheater and destined to do it again...tell your girl and hope she forgives you...thats about all you can do.
by the way, im only tellin you to tell your girl because the girl you cheated with is your baby moms...who will eventually tell her...the next time you cheat (and there WILL be a next time, as I already explained)
DO NOT UNDER ANY CIRCUMSTANCES TELL YOUR GIRL.