I know that feel. For me it's part anxiety and part trauma. You can try drinking some alcohol to dis-inhibit, and it's also helpful if you go with a friend who will be a social wingman for you, like introduce you to people and help drive the conversation towards things you have an easier time talking about, like your interests. If you don't have such a person you will just have to walk up to someone and be awkward. This is more difficult than it sounds, but you will feel great when it actually works, even if it's just a few minutes of conversation before you exhaust yourself. The social muscle needs to be exercised or it will atrophy, and exercise strengthens it. Like any exercise, it is painful when you start up doing it.
I got my start with out-of-highschool socializing by doing offline meets with people who shared my interests. It's easier to talk to someone offline if you have spent some time feeling them out online.
That said, I still do a lot of sitting around when I go to parties, but otherwise I stick around people I know and try to get in a few words with their friends who come talk to them. Networking is one of the easiest ways to increase your number of friends, because you can assume the new person is comfortable with your friend, so there will be some measure of compatibility.