Sounds rather simple-minded, but I found it effective: If you think you have a problem, do something to fix it. If you're fat, start running. If you're stupid, read books (just examples, I'm not calling you fat and stupid). If you don't like parties and consider that a problem, go only with your friends or something, start small. A couple of people in this topic told you to act confident, and really that doesn't cut it. Confidence isn't the problem probably, because true confidence comes from knowledge of the subject or task at hand. If you go to an American football game and you're from Uganda or something where they've never played it, you can't just "act" like you know what you're talking about when it comes to football, see my point? People can usually spot bullshit.
What is the problem more specifically? You said "I feel very nervous when people get into my space." Well, why? Is it because you're afraid that you'll be subjected to public ridicule because of your cleft lip? I don't have one personally so I can't directly relate to what it is you're experiencing, but I imagine that the nervousness is inspired by fear of being made fun of for that feature. Fear of embarassment, more precisely. And the solution to this problem? I'm not quite sure, but I can think of a few things that might help, removing the fear of ridicule, or removing the things that they could ridicule.
1). Man up. Realizing that the people you're afraid of being embarassed by probably aren't worth your time may help. The people you care about are the ones whose opinions matter, and chances are they're not going to be the ones making fun of you. Don't let them get to you.
2). Buff up. go running, lift weights, ect. having a good body isn't the holy grail of life, but it sure is helpful. I started running a few years ago because I realized my habits were not quite healthy and frankly I was overweight, but after a week or two it starts to feel good. In the beginning it's rough, but after you realize you just ran two or three miles when a month ago you could barely get around the block, it feels pretty good, and in a social setting it's something to talk about at least. Join a sport or something, physical health is just as important as mental health, and both can affect the other. Maybe you're a track star already or something though, in which case this isn't applicable to you.
3). Think about the problem more. You gave us a very vague set of clues as to the underlying problem which could possibly be caused by any one of a great number of things. Maybe you can fix it yourself, or maybe you'll realize that you're just being silly, or maybe you'll even realize it has nothing to do with what I'm talking about and you actually do need to see a psychologist or something.
Also, I hope you find this helpful and not offensive in any way. I have made some assumptions which you could be offended by, but my intent is to help, not to harm. And one last thing, SEN may not be the best place for this sort of thing... so take everything everyone has said with a grain of salt...
None.