I feel terrible because I'm in love with someone but she will never feel the same way about me.
That's really sad to hear. Why do you think that she won't feel the same way about you?
I've told her how I feel but she said that she doesn't have those feelings for me.
You should not over give in a situation that is not reciprical. If you feel that she doesn't love you, you should move on and try to let go of those feelings. You can do this by focusing on yourself and your goals.
I'm not over her and I can't give up on my feelings.
I know it's difficult, but trying to let go of those feelings and moving on is the best thing you can do. Obsessing over a person that's told you that they don't feel the same way about you is only going to end up with you feeling terrible.