Why does it hurt to think and look at a girl I like?

I'm 23 years old. This girl I've liked for 8 years now has no interest in being with me. We used to be best friends but now we kind of went our separate ways. I loved her so much and really wanted to make her my future wife. She wanted to be friends and I wanted to be lovers. Every time I brought it up she got weird and we didn't talk for months. We would always go back to being friends but same thing happened every time and our friendship crumbled do to my neediness to want to be with her. Our friendship has transformed to we are friends who rarely talk anymore and it's usually me going out of my way to see how shes doing. And lately if I say I wanted to see her she wouldn't even respond. I think about her almost everyday but I do realize that I need to move on with my life but its weird how I still think about her. I befriended her on Facebook because I found myself becoming a creeper and I knew it was unhealthy. But now I look at her profile that still allows me to view her pictures and I'm happy for her but sad that I'm not with her and all I want is to spend the rest of my life with her but I want her to feel the same way she feels about me. When I see her beautiful face and y body I feel wounded and hurt because we were so close and now our friendship is diminishing. I keep telling myself that we need to get on with our separate lives and we both are but I cant stop thinking about her and checking her Facebook profile. When I used to call her from where I live because I live 2 hours away she really showed no emotion when we would talk. Yes and No answers AKA one worded answers and no enthusiasm like she was happy to hear from me. I know she has no interest in me but I'm ****** stupid. I love this girl and I'd fight for this girl. Should I fight for her or Leave her be? How do I stop hurting Myself and what I mean is if i go years without hurting myself but she shows up back in my life I don't want to hurt myself again meaning after the years of not seeing her? (Sorry for my wording) I know that a potential way to secure this girl is to make allot of money to spoil her and get in the best shape I can get, and become more confident and accomplished. But thats all for her and not for me so I'm not sure thats the best way to live a life. I guess it would benefit me as well. If you have any advice or questions that could help me please ask or give. Thank you.