Littlemiss,
Yes I used to feel that way a lot. In the way I understand it, what happens in these situations is that negative experiences based on past relationships (with uninterested or abusive people from the past) are projected into new people in the present. In other words, even though there is no objective reason to expect people to be rejecting, I would still expect them to judge me, because that was all I had known with my parents and with peers in my past. I had no past experience that would serve as a basis or expectation for people in the present treating me well.

This can get better; it's useful to become aware that what you are experiencing is not 100% "real" - in the sense that usually people are not truly judging you in the way you think they are - that it's coming from your mind as a projection, based on a memory of an unsatisfactory relationship in the past. In psychotherapy, this would be called transference; in everyday life it is a projection.

There is also projective identification; that used to happen to me when I would act awkward or talk to people in ways that made them dislike me. In other words, I made them act in ways that confirmed how I expected/feared them to be. Projective identification involves manipulating someone in the present to act how you expect them to based on the past.

If you can work on understanding these projective processes (which everyone does to a degree) in therapy it can make a big difference. Instead of expecting that people you reject you, you can begin to hope and expect that hey will be accepting and non-judgmental. Because that is how most people are in reality. One of the problems, as long as one is borderline, is that one doesn't fully know that reality emotionally. But it can definitely change.