Your parents are right. You should go to UC San Diego or Berkeley rather than have to fly across the country, pay out of state tuition (which I assume your parents are taking care of) and go to schools with lesser educational resources — all because at age 18, you have feelings of love for a girl who is committed to her boyfriend! 😕
Lots of times people let their feelings get in the way of doing the right thing, and that feeling of love can really cloud your better vision. That, and the fact that you’re “antisocial” as you describe yourself, you may feel a lot safer with a woman who isn’t available to you because you don’t have to expose yourself to social situations which may frighten you. In addition, it sounds like you’re about to leave home for the first time for university life, and that can be very scary for lots of teenagers. In fact, it may be the reason you’re looking for something “safe” to glom onto as you face the big, bad world.
Facing fears is really the only way to get through them. So my advice would be to go to one of the 2 very good California universities that your parents want you to attend, and enjoy your dorm life. Take baby steps attending social functions, and take things slowly. Both of those schools have wonderful social programs, and lots of them. You’ll find your place, although it may take time, and you’ll find lots of women who aren’t already in relationships, but in fact, are ready and willing to spend time with you.
I hope this helps, and I hope you’ll muster up your courage to do the right thing. 🙂