I think there's several factors as to why I can't relax enough to climax with another person touching me (I've been able to climax by masturbating next to another person, but that's it).
I have a hard time concentrating. I tend to have an overactive brain anyway (don't know if it qualifies as racing thoughts or not), but it doesn't bother me so much in everyday life. If something triggers health anxiety in any way, that makes it very hard to relax.
I seem to have a lot of problems with touch. I can cope with it most of the time outside of sex, but sex can be too much. I had an allergy to touch as a child and apparently I didn't like being touched even as a baby (or at least didn't like being swaddled), so it's not a feeling I'm used to. I can cope with hugs and stuff for the most part now unless I'm upset and then it's too much. But during sex, the line between enough stimulation and too much stimulation is really thin and I'm always afraid it's going to be too much or the touch (anywhere on the body) is going to hurt and I can't relax. It hurts to be touched sometimes and I haven't been coping well with hiding that so my partner is getting to the point that he's afraid to touch me again.
I guess I'm lucky to even be able to masturbate next to him and climax, but I just wish that someday I could climax by normal means. It doesn't help the relationship that sex doesn't work the way we'd like it to. It also doesn't help his confidence, as he already thought he was awful at sex and I would leave him because of that before we even met.