Some hypothesis' for the cuckold fetish I've read here and there...

"Claiming" your girlfriend after another man has had sex with her could be like a reassertion of your masculinity or virility.
Similar kinds of situations sort of play out in the animal kingdom... male members of a pack fighting over one female. Biologically speaking, a man who, erm, "flushes out" and kills the sperm of a recent previous male will be asserting and probably securing the success of his genetics. I don't know what the timeframe is like, but the last male is the most likely to impregnate her. A lot of cuckold porn is about the husband finally having her after the other men. It's like being leader of the pack, even though it seems submissive initially. It can also be a turn on or confidence boost that other men find your girlfriend desirable. Other men want her, but she's still your girlfriend at the end of the day.

When the submission and humiliation is a more prominent part of the fetish, well I think that would have the same psychological origins as general BDSM stuff. That's too much to write here but there is a bunch of articles etc on the possible reasons behind those kinks. Maybe find sex-positive kink-positive books to help you understand and hopefully feel better about your fetish?