Need help fantasize a lot

I am suffering from many problems recently. I like this girl. I met her at a gym we used to talk and after we both left we used to chat. She is seeing someone and I can't do anything about it. She will never like me back. I don't know why I like her so much but I do. I have a bad habit of fantasizing things. A future with her. I do it everyday when I go for a walk and I just can't stop thinking about her. I told her I liked her but she didn't feel the same way. She is like the prefect person I would want but I can't have her . This happened with me before I liked a school friend of mine though we weren't close at all I just used to fantasize and couldn't forget about her for Like 2 years. It has been almost a year with my current crush. I need to forget about it but I just can't. I have lost a lot of studying time because of it. Also I feel a lot of sadness in life. I can't appreciate anything good in anyone's life. That started when I got into Batman. I saw the movies some animated and learnt a lot about him. Its like I feel the pain he goes thorough. Though I don't have any reason to. It has ruined my life. I have forgotten how to be happy again. I waste my entire day thinking about bad stuff. Then I decided to forget about her. So I fantasized how I succeeded and in my dreamland she is a sad person because she couldn't have me. I also fantasized about being rich and being Batman where my life is hell. I just can't get those thoughts out of my head. Its made my life really bad.

"I don't know why I like her so much but I do." What about, Because then it means my mind's too taken up with hankering after her and a 'life' with her to get on with what I know I *should* be getting on with doing, including trying to fill whatever gaps exist in my day-to-day life?... as in, carrying around a candle I hold for this/that girl is lot less hassle than tackling my mountain of an in-tray? How do YOU know she's the perfect person? Maybe she eats like a pig, farts and burps like a trooper in bed and-and-and..., and likes to flirt just subtly enough and 'collect desirous male puppies' (fans) which, as her boyfriend, would render you feeling constantly insecure, braced and miserable? My point is, you have NO IDEA what she's like behind closed doors/underneath those surface onion-layers. Closer up and in, she could be an absolute COW for all you know. Define 'hell'.