Your situation is quite complicated. The pattern you describe consists of finding yourself in love with a previous girlfriend, while your current girlfriend feels like second fiddle and dumps you. Then, you go on to the next girlfriend, and the pattern starts all over again.
What you need to do is understand why the pattern exists and what purpose it serves (realize that no pattern exists unless it meets an emotional need, albeit an unconscious need, of which you may not be aware). The outcome of the pattern is that you end up feeling like a terrible person who lets everyone down. Ask yourself why you need to put yourself in this position.
The fact that you always end up feeling this way makes me wonder if you are actually familiar with this role. Did your parents make you feel like a terrible person who always let others down? Or is it possible that you grew up with a parent who let you down? If this is so, then you may have patterned your behavior after that parent, which could explain why you let others down the way you were let down. In doing so, your unconscious mind actually purges itself of the pain you suffered as a kid.
Now, you are the one who does the letting down; you deposit onto your girlfriends the bad feeling that you felt as a kid, and you feel freed from the pain. In the end, you still suffer. While your unconscious mind may think that it is freeing you from being the one who is let down, you are constantly feeling let down because you can't create lasting love. You are alone and miserable all the same.
While we are exploring, also examine how not being able to let go of an ex relates to your history. Did someone leave you when you were young? A parent, perhaps? This might explain why you find it hard to let go. It would also explain why you keep replaying the pattern. Repetition of a trauma is a means of working through the feelings attached to that trauma. Each time a relationship ends, you find yourself yearning after someone who is gone and who you can't have.
Sound familiar? Did you live this as a child? While the repetition is designed to help you work-through the pain, it also prevents you from enjoying the love that is available to you in the present, meaning that you are never free to experience love. I think you need to figure out what piece of your childhood is being played out through this repetition. Remember, when the mind backs you in to a repetition, it is trying to heal a childhood wound (see my Advice Archives under repetition compulsion).
When you understand what wound you are trying to heal, and you work through the feelings tied up with that wound, you will not need to keep recreating this pattern over and over again.