For the most part I would say staying friends with an ex is ok and certainly possible. I still talk to mine 3 years later. However, my friend, YOUR gf is not just talking to him, she is getting drunk with him, spending a lot of time with him, not answering your calls when she is out with him?? Sleeping over at his place? Major red flags. Sorry buddy, but she is playing you like a cheap violin.....I think you see it but you are in a bit of denial. You may love her, but she clearly does not have the same level of love and respect for you, and you deserve better.