I'm sorry this is bothering you. Of course I don't have to say there isn't any logic to you feeling this way, you married the girl and therefore won over any other guy. But let's look at the whole situation. Some questions:
She has only had two partners in her life? Or, you aren't bothered by any person she might have dated before you met, but are just by this guy? Was the breakup of your choosing? Did you feel (emotionally) that you were really broken up? Obviously if you did think you were broken up, you wouldn't have felt any ownership of her, in fact guys who don''t want to be with someone often feel relief when they break up. If you were in that position, some new guy would have been an ally in getting the clingy ex-girlfriend off your back. Clearly you didn't see it like that, though.
If you two were really broken up and both of you were sure that was it, well, a breakup means you don't have any claims on each other and are free to get into relationships and sleep with someone else. Did you sleep with anyone else in that time period? Is she jealous? Sauce for the goose is sauce for the gander, as the saying goes. If you slept with anyone else in the interim, you don't have lots of justification for being bothered by what she did when you two were broken up.
You two then got super lucky, and realized you really wanted to be together. It's kind of romantic -- a Hallmark movie could be made out of this. But you now have visceral jealousy, or competition feeling, or comparison in your mind,. Is it only focused on this guy? If she has had other boyfriends in the past, if this guy didn't exist would your jealousy focus on one of them instead? If you're not jealous of past boyfriends but are jealous about this one, what was going on with the way she got together with this one? Did you think she got together with him before you broke up or something like that? In other words, did you feel somehow fooled, not just broken up? I could see that would leave some amount of obsession over the incident in your mind.
It's pretty normal for guys to feel jealous or possessive, but maybe a little less normal when they are the one who basically won the game by getting the girl. Do you wonder if she is not certain you're the right guy? (I don't mean by benign actions like still being on good terms with her exes -- if she hated them, that would be a bad sign, it would mean she had terrible taste in men.) I just mean, is she enthusiastic about being your wife?
Anyway, if you think it's merely garden-variety jealously, one way to deal with it is to remember it's a primitive emotion, and not to let it run you. Just deliberately choke off any mental images any time you have them, until you stop having them. Dwelling on it, particularly on one person who probably doesn't deserve you to be so mad at him, will take you only to dead-end self-torture land. She's had other partners, so what? They should be anonymous to you. It makes life easier -- millions of adults who are now married had partners before they got married. If we couldn't cope with that, none of us would be able to get married with a whole heart, and that is the goal of getting married.
If you think there is something more to your feelings besides just jealousy, like you are getting signals from her that she is regretful of the marriage, go in for counseling together. Even if not, you could consider going alone. It can be a huge relief to tell your troubles to a therapist or counselor.
My guess is, she loves you or she wouldn't have married you, and you've just hooked yourself on something about the situation, but it doesn't mean anything about her love for you. Tell your mind to knock it off, and love her back.