It's okay to cry. I think you should talk to a professional about this since it is causing you so much anxiety that you are hurting yourself. Are you seeing anyone?

Being with other guys sexually doesn't mean she'll cheat on you. It just means that she's been with other guys and that can be a good thing. She's got some experience and to be honest, I think a less experienced girl is going to be more interested in doing sexual stuff with other men because it's newer and more exciting. But if she didn't like you, she wouldn't be with you. Plain and simple.

As far as telling her goes, if you two have been together for a while and you value an open and honest relationship, I think you should tell her. However, before you do, you have to recognize that there is nothing she's going to be able to do or say to take your anxieties away. She can't go back and change what she did in the past. The problem isn't her, the problem is your self esteem/anxiety/insecurities/trust issues or whatever. She can't prove her loyalty strongly enough to totally abolish all of your anxieties and expecting her to do so is unfair. If you go to her and tell her about this, I think you need to be open to the idea of getting help outside of the relationship to work on yourself. If you don't, your fear of her cheating will eventually destroy your relationship in all kinds of dramatic and creative ways.