Be firm with him. Smack him upside the head and tell him how much you love him and that you want to be with him, not some other guy.

My bf used to do that kind of thing to me, and I hated it. At first I felt bad and sympathetic because he kept saying, "You'll be happier with somebody else.. I don't deserve you." Then after a while I told him to shut up and just listen to me when I say that I love him.

I say to do whatever you think will make you happiest. Where are you in school? I'm sure you still have some time to decide what to do. If things are still going wonderful with him and you would like to marry him some day, ask yourself if that specific career is worth giving him up for. Then ask if you're willing to risk not getting into the field you want most just because of a boy. The general rule is that you shouldn't plan your life around a guy, but I think it varies from case to case.