My first girlfriend says our relationship has run its course? I still love her so how do we get back together?

I am 26 and my partner of 2 years is 20. We met at my old job when she was seeing someone. We hit it off but she didnt push anything because she had a boyfriend, but her boyfriend only saw her as a friend with benifits. Eventually she discovered she had feelings for me and within a month we were in a relationship. Her ex caused problems at first and warned me she'd do this to me, but I was different. She has never been in a relationship as long as us, and were not your average couple. After being together 4 months we went backpacking Europe for nearly a year. Throughout that time we'd never argue, great sex, and very much in love. After Europe she wanted to move out of our parents and get our own place. It took us a year to find new jobs and get an apartment, but everyday she said she loved me and talked about marriage and kids. We now work in the same place and everything was going great. It was after 3 weeks of living together i noticed a change in her. She always used to cuddle into me as we slept, but then found I was the one always doing it and she'd push me away a lot. Next she started having a more active social life, where in the past she was quite happy to snuggle up together and stay in. At work I heard she had lied about me to her friends saying I didn't want her going out and I was controlling. I confronted her on this and she told me all her friends were lying? Lastly the final 2 weeks she started hanging out with a coworker an awful lot. He is fat and ugly and I didn't feel threatend by him, but I noticed they would go the gym together, text between 15 - 20 times a day & he'd take her to work socials. I went to many socials and got the vibe that he wanted my girlfriend. I warned her he was advancing on her but she promised nothing was going on. The final nite we were together she promised me we would have a romantic evening together just me and her when I finished work. I called her 15 mins before the end of my shift to say I might be a bit late, and she asked if it was ok if she went out! I said please dont go out as tonite was about us, but she insisted and told me she was getting picked up and when I get home she would be gone. I told her that we really need to talk as I think our relationship is in trouble, but she said dont be daft and would be home early. She walks in at 4am drunk and I just burst into tears. She shows no emotion as I say our we over and she says the relationship has run its course! Not once since the breakup has she cried while I've been a total reck. I made her move out of the apartment that I got for her and she went to stay with a friend. We had a good talk soon after and I was lead to believe there was still hope if I gave her space to think. The next morning I went to her friends to drop some of her belongings on the doorstep and was mortified to see my girlfriends car parked in fornt of the ugly coworkers. She doesn't know I saw this, and her friends keep saying probably nothing happened, but I just don't know what to believe anymore. I've done everything this girl has wanted and she says there could still be a future, but I just don't know now. I love her so much but i'm at a complete loss as to what to do???