I have experience with this. The psychoanalytic therapy that I've been in to help with these kinds of things has helped me to discover that this is an internal issue about feeling overwhelmed, about a fear of being engulfed by others that I might 'let in' even a little. I've learned in therapy how to be myself more which makes others feel less threatening because a strong core is a defense against the feeling that others can overwhelm me, hurt me, make me do things I don't want to do.

I still like my quiet time and activities and I can make room for both.

The more you can keep practicing getting put there, the more familiar that will become to you. Then you'll have that feeling of familiarity that you are wanting. Good for you for practicing.

Talk to your boyfriend to work out a way for you to be able to leave events earlier when you feel too overwhelmed to stay. If you are with a group you can claim to have another commitment (that commitment being to take care of yourself, to soothe your own fears, to feel better). When it is just the two of you, try going the places you can during less crowded and busy times until doing that becomes familiar. In my experience, feeling familiar with a place takes away a level of fear (anxiety) so that other things, like more people, doesn't crank up the anxiety (fear) to such an uncomfortable level. It feels better.

Get a good psychoanalytic psychotherapist to help you with this. You will learn a lot about yourself and you'll be supported along the way. You can do this!