I really need advice

So me and my SO broke up about a month ago after being together almost 2 years. He is a marine and when he came home from boot camp he proposed to me the day before he left. Then he left for the second part of training and they don't get their phones during the training and when the training ended he got his phone back and he was just acting really weird. We hadn't talked in a month and the first text I get was I failed one of my tests and almost didn't past. He was just really short and we barley talked. This went on for a week. Then I tell him I can't do it if he's gonna act like this forever and Mabye we should take a break and he agreed but then I get a phone call the next day saying we should just break up and he needed time to focus and he doesn't "feel right" right now. I was very confused because we have always been "the perfect couple" he was always my best fiend and we know every detail about each other. So I of coarse being me begged him to come back and in oils try to just help him in anyway and he kept saying no. And I kept asking him why and finally the next day he told me their was someone else. He said they have never done anything but kiss a couple times but he never really cheated and I wasn't mad about the kissing cause he couldn't tell me cause he didn't have his phone. But the day we broke up they started dating. They are currently at mos school but they are at different schools. I keep telling myself it's not going to work between them because this girl does not seem like his type at all. Over the fourth they have a 96 (he has leave time) and he is going to fly and meet her and meet her parents so I guess it's pretty serious. We have talked everyday since we broke up be used we have been best friends for 9 years. He always told me I was the one and he would never find anything better and all this other stuff plus we were engaged which makes it so much harder. I keep telling him it's okay we all deserve true love but I keep hoping he will come back because I can't see myself with anyone else and it kills me just the thought of them being together. He text my mom asking her if he was allowed to put in a relationship with this new girl because he didn't want to hurt me and a month later he is still apologizing. He still comes to me with all of his problems and has said several times that he can't talk to his new gf about it and how I help him. Is this just a phase he's going through?? I have tried moving on and I have had sex with other guys just trying to get my mind off of it which is bad and somedays I don't give it any thought but others I just want to cry and I get so depressed. They haven't had a relationship outside of a military setting so I keep hopin once they are on their own they won't make it. But I just want him happy. It's just confusing cause he still says he loves me and when I say we shouldn't talk anymore he begs for me to keep talking to him and he cares so much what is wrong. My sisters husband recently tried to rape me and he was so concerned about it it's just all so confusing. Has anyone else ever had this happen or is in the same boat. I need someone to relate to and talk to about it because I can't talk to anyone about it but it's really getting to me today. Plus this new girls looks a lot like me just manlier. I also don't want him getting hurt because this new girls recently broke up with her bf and I feel like she is using him as a rebound because she's flaunting their relationship. Please help