Ex Boyfriend Help

My current girlfriends ex boyfriend and I have a very intense relationship, be basically just stay out of each others paths as best as possible and we both don't want to cross paths again. My current girl friend of about 9 1/2 months 💓💓 decided it would be a good idea if they (the ex boyfriend and her) hang out tomorrow after work for a couple hours. (Keep in mind that they would be alone.) She has done countless things for me to lose her trust, including what I see as cheating on me (She ended up being in an online relationship with some random kid and they would sext each other, but this was in the beginning of our relationship so idk about that) ANYWAY, I'm not comfortable like, at all with her hanging out with him alone so I decided I would. tag along.. I really just don't want her to go with him instead of me because I had major trust issues with her because of the things she has done to me in the past. Also the ex boyfriend talks to her consistently and when I say consistently I mean at least 3-4 conversations a day. She will even go to the point of breaking up with me just to hang out with him today. I've tried everything I she just won't crack.. I feel like I have no say in what she does. but when it comes to her, if she told me what to do then I would do it in an instant. Call it a stretch but I feel as if she hasn't gotten over him in the 9 months we have been dating. I'm s scared of losing my girlfriend. I need help on either, how to trust somebody or how to get her ex out of her life.