Why am I so terrible at holding relationships of any kind? Am I just a naturally unsociable person?

I'm like that too, for me its kind of like my defense mechanism, I've had a lot of people burn me in life so whenever someone starts to get too close a warning bell goes off in my head and I just let the relationship fall apart. I hold people at arms length and sometimes it feels like theres nothing I can do about it. I hate that, but I also have a low self esteem and I feel like maybe I will end up disappointing this person because they'll realize that I am not as cool as I seem on the surface, and they'll end up being the one doing the leaving so I do it first. When I was at my most confident it was because I had something that made me proud of myself, something that I did well that most others couldn't do, also when I was healthy and when I respected myself.