How do we let go?

For some reason I can't let go. I don't want to give up on something that all signs point to "he will never be mine" Why is this a hard concept to grasp? There are people I have dated that I don't care if they are with me or not. So in almost all my relationships I am the Strong independent person that I have worked so hard to become. But...There have been 2 men that I have completely fallen for and for some reason I am so scared they are going to leave. I transform to the girl that will do anything for them, the girl that gets walked over. I need to find a way to stay the independent woman that i am not matter how much I fall for a man.