How to I be more outgoing around a group of people?

I've always been a huge introvert. I'm an only child, and grew up very shy with little social interaction. Everyone said I would grow out of it, but at 19, I never did. I was bullied throughout school for being the quiet, "creepy" kid. I prefer to keep to myself, and have only a small group of friends. It takes me ages to be comfortable with the people I meet. I'm 19 now, and my boyfriend is always wanting me to go to parties with him, hang out at his friends' house, and social things like that. I always suggest just spending alone time together, to get out of it. The last time I was at his house, his room mate invited a few friends over, and I had a serious anxiety attack and couldn't talk to any of them. I ended up having to go outside and take a long walk alone to stop panicking. I know I need to open up some, or I'm eventually going to lose the most important person to me. I don't know how to just open up. It seems like it would be easy, but I have very poor social skills. Sometimes when I try to be social, my words come out totally wrong and I end up looking stupid. How can I be more comfortable with people and open up? Please don't suggest medication or a therapist...