How can you conquer it? Talk to your counselor about it. Tell her that you feel threatened in bed by her past sexual relationship. Tell her that you're worried that you have a small penis and that you don't satisfy her.
Because, basically that's what you're saying to us.
You're not angry that she slept with someone. You're angry because you feel threatened. You're angry because someone else might possibly compete for her attention on a sexual level during encounters.
So what do you do? Well, you could rage impotently about it until the entire relationship self-destructs out from under you because you're a moron... Or you could talk to your counselor about it. You could get some books to educate yourself about sex. (Start with the ultimate guide to cunnilingus for a nice physiological overview of female arousal and stimulation.) You could start an open and running dialogue with your girlfriend about how you feel. You could talk to her about sex, what she likes, how you could do better. You could you know, step up to the plate and actually put in the effort to become the most amazing lover she's ever had.
Or, you could develop some sort of cuckhold complex that forces you to sit in the corner and toss yourself off why she sleeps with a real man that actually is capable of pleasing her.
Do yourself a favor, become her dream man in bed. Become the embodiment of sexual energy in her mind. Don't ever say, "well it worked for other women" or some other bullshit line that blames your inability to please her on her. Be THE MAN in bed. Take charge when it comes to pleasing her, not just getting your rocks off. Make her goddamn toes curl. Make her tell her friends about you until their cheeks turn pink.
Seriously. Be a goddamn man and actually CARE about pleasing your women. Once you do this, you don't have to feel threatened by anyone she's ever slept with, because frankly, they'll never compare to you.
And even if that relationship ends, the next woman you find will worship you like a god for putting in the effort to actually be worth a damn in bed.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."