I have a tendency to hang onto relationships for far too long, whether it's a flaky friendship or a romance gone bad. It's difficult to let go of something that began with such hope and enjoyment.
What usually happens is the other person has already moved on while I'm doing all the work of the relationship. Then, when I finally stop, I realize that they never even notice the change.
The plus side is that when I have stopped my life improves. When a source of happiness becomes something depressing and stressful, stepping away from it is the best thing I can do for me and everyone around me.