Only 24 and Lost Everything Because of Myself

I recently turned 24 years old and feel like an old man. I recently left college and moved back in with my parents. I was madly in love with my girlfriend, we had talked about getting married and had imagined an elaborate future together, and one day out of the blue, she dumped me. We were best friends, I didn't understand it. I was very confused, sad, hurt, and angry, and called and texted her daily saying can we just talk about this...she would ignore me and tell me to leave her alone and that she needed/wanted space. By the way she dumped me 2 days before my birthday, it was a lonely one to say the least. She didn't get me anything. Anyway, after two months of holing myself up in my apartment, only going to work then returning home and drinking lots of beer or whatever else to fill my days, not sleeping, etc...I came to find out that there was another guy involved. It was this guy she used to work with. Once I found her out and confronted her, she admitted that he tried to kiss her about a week before we broke up but swore that he had nothing to do with her decision, even though she admitted she had a 'small' crush on him. I think what happened is he led her on because he wanted sex and only sex from her, and she thought there was a chance for a relationship with this better looking guy, so she dumped me just like that. Now they don't even talk, I think he realized she really liked him and wouldn't put out without a relationship so he backed off. The whole time she was ignoring my calls and if she answered she would rudely tell me to leave her alone. She used to be so sweet to me and cared about me and then just suddenly overnight turned into an unemotional monster. I didn't do a damn thing. After I found out the truth, I got really drunk one night and went out to my car, put in a CD she gave me, and put a vacuum cleaner hose in the tailpipe fit snugly with a t-shirt and put the other end going in a window. I tried to kill myself. I started to get really dizzy, freaked out realizing what I was doing, and called my parents because I felt bad. They told me to stop and to come back home the next day so I did and I'm still here. I have no friends here, they have all gone off to college and have jobs and I feel like a loser living with my parents and pining over some 21 year old girl that broke my heart. I think of her and look at her facebook page constantly and have to force myself not to call her. I thought she was such a good person, my best friend in the world, and she hurt me more than anyone ever has and it doesn't seem to bother her. Also the whole time we were together she didn't really have any friends, she only hung out with me and my friends, then she got a new job and finally got some girl friends and then a month later dumps me. Why? I don't get it. Was she using me because she didn't have anyone else?