She had announced that her and my boyfriend have been together for months and that he has cheated on her before

Well to start off, my boyfriend is an active duty Marine. There is a slight age difference, so as of now, we must be kept a secret. I met him on a dating website a few months back, we went to dinner and a movie, and hit it off right away! It was great! He was like my dream guy, said & did all the right things! We had only been together a month when I got a call from a number in a different state. I have no friends from that area, or anyone I even slightly know. Anyway, the girl on the phone was very sweet! Which under the circumstances, I didn't expect. She had announced that her and my boyfriend have been together for months and that he has cheated on her before, many times, but she was too dumb to leave him, and she didn't want me to make the same mistake, to leave before we got too serious. Of course, I had to talk to my boyfriend about this, so the next morning, he had called me, I explained the situation. Like any other guy would, he denied it, but I ended it anyway. I have trust issues with men to begin with, but this was just too much. Yet, he is still trying to get back together, swearing he never cheated and that it was "one of my friends trying to break us up." When like I stated at the beginning, our relationship was a secret! The most my friends knew was his first name and that he was older. Nothing else and this chick had a lot of information on him that I didn't even know! I do still have feelings for him, but I can't allow myself to be so stupid as to take some man back who may have cheated on me! Honestly, I just need some advice from someone who may at least slightly understand my situation. It'd help a lot, and I would really appreciate it! Thank you, ahead of time for taking time out of your day to read this and help me out. <3