It may be that you are naturally more introverted than your boyfriend, or it may that, lacking support, you are developing social anxiety. It sounds to me like you have now fallen into a negative cycle that is making things worse, not better. It goes like this: You aren’t comfortable with people, so you withdraw more and more, which means you don’t develop the skills you need to get comfortable with people, which makes you withdraw — and so it goes.
The incident at the party only confirmed your worst suspicions about being out in the social world. You ran into some bullies. Yes, bullies. Your boyfriend’s so-called friends were terrible to you. But that doesn’t mean that they represent the whole world. Clearly your boyfriend doesn’t care about your size. Neither does your friend. Why should the bullies’ opinions count more?
You can learn how to be comfortable with people. You can learn how to handle bullies. But you probably can’t do it on your own. Your fears have become too great. I therefore strongly urge you to find a therapist who specializes in anxiety disorders. With guidance and support, you can overcome your fears.
I wish you well.