Hello and thank you for your question.
This has become a cycle that you are both playing into at this point. It really sounds like the "other guy" is someone that is convenient for her to run to, when you and your girlfriend are having problems. Also, most of your problems are now because of her involvement with this other guy. Who she then turns to, when you have a fight. So it's like a circle that needs you to break this pattern. It also sounds like she may not be completely honest about her relationship with this ex boyfriend and that she is actually with both of you at the same time. Even if he is just a "friend" that she is just having fun with, she is disrespecting your feelings about him in her life, and she needs to stop seeing him altogether, if you and she are going to work on your relationship.
What you need to do, is be the one to break this circle. You do that by not asking her to hang out with you for example, this weekend. You tell her that until she is ready to see you and only you, and also let this ex boyfriend go, that you cannot see her. She needs to make a choice and again, needs to understand that even if he is just a friend, her involvement with him, is causing problems in your relationship. That he has been more than a friend to her in the past, and even recently she had admitted to still having feelings for him. This situation is one that you don't need to tolerate, even if you have your own things to improve. So again, be firm and simply tell her that you want to be with her and have a good relationship, but can only do this if she will stop seeing him.
You see, every time she runs to him when you two have an argument, it breaks down your relationship and bond with her. She needs to work it out with you, not run away to him. This is just one example of why this triangle is destructive. Again, it's important that if you do set this boundary with her that you follow through,otherwise she will walk over you even more. Please click ACCEPT, if satisfied. Or ask for more info. Thank you!