How can I stop thinking about someone who doesn't like me back?

Think about why you're thinking about them not liking you back and think about why you want them to like you back in the first place. When you get those answers then think about if is worth thinking so much about them or not. Depending on what you thought about you can decide what to about it

either by searching for someone else to replace her completely from your mind or by doing things that'll help you grow.

I've been in a similar situation before. Instead of dwelling on the person's feelings towards me (or lack there of I should say) I tried to write down everything I felt towards that person; things I liked about them and things that may have annoyed me. This helped me to clear my head from thinking about them too much, and i felt so much lighter after dumping everything down. One other thing I did that may not help everyone is I took those words and turned them into a song. For me personally, music is my way of life and I found it beneficial to say goodbye to my heartache that way.