It’s a little sad to be excluded, but this happens a lot, and in the adult world, things like a boyfriend, having children, work, school, etc. will come between friends. Part of growing up is learning how to prepare for how to handle this eventuality. It doesn’t mean the friendship ends.
You have two choices: you can either mope about it and try to “get your friend back” which will make you look desperate, and push your friend away even more, or you can try to make more friends, which includes girl2. And girl3, girl4, etc. Do you have a cell phone or iPod? Remember how when you first got it, you thought it was the coolest thing ever? Making a new friend is like that, and your girl1 may be a little obsessed with girl2 because the newness of the relationship.
Take up some new interests and hobbies to feel good about yourself, and meet new people.. Resist the urge to stay in your room and talk about both of them on myspace or facebook. Don’t buy into the drama or create more drama. Get out and do something real and interesting. Join a group, either at your school or in the community. Do volunteer work.