You know what your problem is?
You get attached, fast. And once you're attached to someone, you do everything you can to please them and make them happy. It's never been about what you want, it's always everyone's needs before your own. You give out too many chances to people, who quite frankly, do not deserve them. They take advantage of you, and you become a pushover. But you're okay with that, because they're in your life and that's all you ever really wanted. And even if they screw you over, you'll still be there for them. Because that's you, that who you are. Once you get attached to someone, they capture your heart and they always have a place there.
And that is why it's so hard for you to let them go.