Why am I so obsessed with someone who clearly isn’t good for me?

I’ve obsessed over a girl for over a year now and we’ve been in and out of relationships constantly. She’s chosen another guy over me, lied straight to my face, gaslights me, ignores me, love bombs me and makes me just desperate and miserable. She keeps me around and tries to convince me she wants to be with me when she won’t try to even hangout with me. I keep beating myself over saying it’s my fault it’s like this because I’m scared to say things and that I deserve it. It’s so bad she can say one nice thing and I just forgive her and follow her blindly, I need to stop somehow and focus on my mental health but I don’t want to be alone.