Why am I still having dreams about her?

So this girl (I thought I loved her) and I started out with excellent relations; we would text, hang out, hug, etc... (she was not my girlfriend, though I wished she was). Then as time went on our relationship went from good to bad because I figured out that she was manipulating me, using me, leading me on, and she chose a guy over me 3 times. I don't know if her intentions for keeping me around were good or not. She told me that she didn't want to lose me, but she still chose another guy over me 3 times. So I ended our relationship for good. I said my goodbye and left in anger and haven't seen or spoken to her since. That was months ago. For the month or two, all I thought about was her; then I stopped thinking about her as much; then recently, as in this week, I've started to dream about her like I used to (not wet dreams). Why is this? I thought I loved her and she me. I'm just so confused. Now I'm getting thoughts about wanting her back. Can I have some advice on what I should do please? Should I try to forget about her, go after her again (fourth time), or what?