I think she is in love with someone else

Don't act all clingy in the hopes she stays with you, your going to fell much better if you accept a breakup when it happens, ask her honestly if she has found someone new, if so move on, see your friends more, make new friends, take up a hobby, you'll feel better soon.

First, are you guys together? How long?

And then, why do you think she may love someone else? Have you talked to her about this?

More info!

move on and feel stronger... girls don't like needy clingy guys... it might not be the right time now.. and she might not be the right girl but later on you will find the right one ^^

Well first off we need way more information, but I think all you need to do is take a step back and look at the situation. You need to non-clingy-needily ask her about it, if she is in love with someone else then there is little you can do, and the more you fight the sooner it will happen so just let go, move on and be strong.
If on the other hand she isn't in love with another and you misread some signals, then be prepared for an argument about trust, just honestly explain how you came to the conclusions you did without yelling or crying, and she should see your point of view (although will still be annoyed).
So there you go you're covered for both scenarios