First, good to hear you're out of this relationship. Kicking yourself over past issues never solves things. Learn from it, look within, and grow. Something to keep in mind. You're out of the relationship physically. It sounds like you're still in it mentally, still giving him power over you. Stop it. So what if you run into him and his latest conquest? Living in fear over that you might run into him at a tournament is to still give him power over you. You might run into him at the mall too, or the gas station, or the park, or the grocery store. So you either accept this and as they say "armor up" or stay home, lock your doors and pull the covers up over your head. I don't think you'll be doing the later dear.
First, accept you made a mistake. A few actually. Accept that.
Now learn from it, and don't do that again.
Second, don't keep revisiting it. Living in the past continues to give him power over you.
Move on.
Third, go back to karate, go back to training, and associate with positive people who will build you up, not tear you down.
Take it from one of the biggest *** holes on this site, if you keep letting the victimizers hold power over you, you are destined to go from bastard to bastard to bastard until you die, explode or decide that a small room and 50 cats is all you need. Break the cycle of abuse and powerlessness. Seek some professional help if needed.
You can do it, it's just going to take work and time and a few good friends, and a lot of personal effort. You have some good advice in these posts. Learn and grow. Relationships should be of equals, not master and slave.
Well, not unless you're into that sort of thing, but I mean in the good way, not this way.
Good luck.