Is it possibly an issue with additional anxiety on test day? That was what undermined my initial med school effort in the States, and it can be a real bear to manage. I would recommend talking it through with a faculty (or other respected) advisor who has your best interests at heart, and possibly look into reasons for this discrepancy.
(On the good side, if that's what it is, there is certainly hope. I went through an excellent anxiety treatment program in New England that helped a ton, and I am now back to my old self when it comes to exams... as long as I am fully prepared, of course!)
If you still want to be a physician, then do everything in your power to identify the problem, and to rectify it. If you don't really want to be a doc, then now's as good a time as any to stop beating yourself up for something you hate. However, if you are scoring well on UW and NBME, then there is really no reason for you not to perform sufficiently well on the Step. Assuming everything is as described above, it would seem that the raw stuff is there, you just need to access it. Good luck, huh?
"To array a man's will against his sickness is the supreme art of medicine."
- Henry Ward Beecher