Why do I feel this way about an ex?

I'm sort of venting here and I apolagize that this is long, I just want to know if someone out their has felt this way, and what have you done to get over that ex that has made an impact similar to this?

In 2006 I was 18 and a senior in high school. I met this guy and had totally fallen for him. I had dated jerks in the past, but he was different. I loved him. Three months into the relationship my mom committed suicide. At that point I had lost everything, my dad had died two years prior and I was suddenly without a home and a 16 year old sister to take care of. I felt like I went from 18 to 25 years old over night. The guy I dated was sort of my rock. I never complained about what was given to me, I sucked it up and tried my hardest to keep a float. Three months after my mom died he dumped me through a text and again I felt abandoned by everyone I ever loved. We never talked about why he dumped me, we never saw each other after that except in passing, he never so much has even asked how I"m doing. All of a sudden it's like a jerk switch turned on. I spent the next 4 years going from house to house to house, never staying in one place for more than a month or two. The little bit of family I do have is very uncaring, if I"m not handing them money they dont want to help me. I've never done drugs and I dont go out of my way to hurt people. I dont sleep around, I work ALL THE TIME! Long story short the last 4 to 5 years of my life have been a living nightmare and through this time i've never been able to let go of that ex. He ignores me when I try to talk to him, he drives by, stares at me and doesn't wave or even really aknowledge me. When I see him in public he'll glance over but no smile, no wave, nothing. My friend is currently engaged to him, and at one point I was a friend, now I"m reffered to as the ex. lol I miss him, he was so nice to me and I dont know what I did wrong to deserve all this. The little bit of family I have doesn't care about me, my mom didn't care about me and he didn't.............I dont feel like this all the time, but every once in a while I feel like I"m just not good enough, I get depressed. The saddest thing of all is that I have a fiance ( been together for a year) who truly is an amazing person, but as hard as it is to try and see him being the father of my kids, or us buying a house and getting married, I can't picture it. Is this normal? My friend who is engaged to my ex will call me and complain about him when she's drunk. Appearently he's mean to her and she's a ***** to him, when she does this I feel so relieved i'm not with him because I have a wonderful fiance, right? But every once in a blue moon, like today. I'll miss him, and I'll want him. why? He isn't even that great of a guy.