I have a friend who has a similar problem to you, she is okay around me because she has learned that I am someone who she can trust, because I have helped her on many occasions, but we have fallen out many times over stupid little things because we are both paranoid, but we always make up and somehow we have become the best of friends because we are both just like each other I suppose. The main problem why my friend finds it hard to trust people is because she has insecurites about herself such as her weight and sometimes she will twist the words of others and she gets it into her head that they have said something to insult her when they have said something completely different and even she will see a compliment as an insult if it is anything to do with appearance.
You want to make friends but because you find it hard to trust people it is making it difficult for you. The whole point of friendship is that you need to find people who you can trust and who you are happy being around, but you need to give people a chance to shine, sometimes people who seem to be the average joe can often turn out to be some of the nicest people that you have ever met. You need to stop being so tense and have confidence in yourself so that you can be confident around others. You say that you have a good set of friends already and that you want to be close to them. You need to make the effort to get out more. You seem to be someone who would benefit by having just a few, but very close friends who can be with you whenever you feel that you need them for support or company. I think that there is something deeper than the surface problem and there is only you who knows what that problem is. My friend's deeper problem was that she had been psychologically affected by some events at home and this had led to her feeling depressed and had even affected her studies. She became like a hermit and wouldn't go out as much because she was so upset and she felt that if she went out she would fail her coursework through enjoying herself, although she was doing very well after a couple of weeks. You need to make more effort to keep in contact with your friends, as by ignoring them you are making them feel unwanted, as a result they will fall out with you most likely. If someone breaks contact with you, the other party involved wonders why. You need to talk to your friends so that you know whether you do truly want to hang around with them, and as I said before they may be totally different people from the visions that you have of them. By ignoring them you are only making yourself feel worse, so what harm would it do to spend more time with them. If you want to stay in, why don't you invite a friend round to your house to watch a dvd, which means you can chill out, but still have someone around to talk to. You are entitled to your own private space and you should tell your friends that you want to be alone sometimes. If you explain this to them then this will separate the true friends from the less trustworthy, the best friends will respect your privacy.