OP: looks like your tally of girls not being faithful is about to be 3, if not already. Hint: it is not all on them. A good part of this pattern is on you.
In a nutshell, you put too much effort into your relationships.
Women are naturally attracted to resourceful men who lead them. If you care too much you cannot be an effective leader to your woman and you will not have a woman.
The guy she is seeing cares about her one tenth as much as you care and yet he has her interest not you.
Your only slim chance to turn this around is to ghost her, go no contact, and focus on yourself. If she chases you after the no contact, great. Let her chase but put very little effort into things. Make her do most of the work.
If she doesn’t chase… well you quickly learn her true feelings. Move on.
A relationship is like a movie.
Your dating strategy is to go straight to the end of the movie. This is a turn off for women.
The best parts of the relationship are the beginning and middle parts of the movie where the woman is not sure if she can win the guy.
The end of the movie gets kind of boring because you know the rest of the story and that is less fun.
Best of luck to you!