HELP!! WORST FEELING IN THE WORLD!?

You really need to grow up. He is his own person and he has the right to see whomever he wants. If she is really that bad then eventually he will see that in her and dump her. But as of right now, you haven't said a word to him and you're mad at him for seeing someone else? I think you need to take care of your jelousy problems before you even begin seeing guys in the first place.