Start giving a shit about what you think of you. If that's hard, it's because you've not really thought about it before. Once you've got your own back, nobody else really matters.

Also talk to people. Talk to them all the time. Standing in line at the store. Passing by on the street. Just make an observation about something to start the conversation. Compliment someone. Over time you'll get two things from this. First, you'll realize that everyone is just a person like you (like, the subconscious part of your brain will get the message). Second, you'll get a bunch of little rejections (weird looks, things like that) over time in a scenario where you basically have no skin in the game. You'll no longer care about rejection and you'll realize that people rejecting you almost never has to do with you - it has to do with them.