The day has finally arrived, the day you have been waiting for: your ex is asking you if you want to get back together. Everything seems to be on the right track until he (or she) suddenly (and quite appropriately, given that you are going to sleep together) mentions the people he has had sex with while you were broken up (or in some cases, he does a little more than mention them—he elaborates on the details of his sexual encounters).
You know rationally that your guy didn't cheat on you. After all, you were not together when it happened. But your fantasies and images of your ex together with others haunt you—they make you sick to your stomach. It may even prevent you from acting in a sane way, and your jealousy could abruptly put a complete stop to the reunion with your ex.
There is only one thing for you to do in this situation. If you really want to get back with your ex, you have to block out those images and keep telling yourself that you were not cheated on, that your ex had every right to do what he did.
But why is it so painful to think about your ex's sexual encounters, when you won him back in the end after all? Here are five possible reasons your heart is still aching.

1. A breakup can cause grief. Grief is a complex emotion consisting of more basic bodily and cognitive emotions, such as surprise, disgust, denial, sadness, anger and acceptance. These stages do not necessarily occur in this order, and they may repeat themselves multiple times. To get through grief, you will eventually need to reach the stage of acceptance, in this case: acceptance that your relationship is over. This tends to be the hardest part for most people who were at the receiving end of a breakup. Most people who are involuntarily broken up with do not accept the breakup. They still believe (or at the very least hope) that things will work out in the end. If you haven't accepted that the relationship is over, then there is a part of you that still considers the two of you in a relationship (however irrational that can seem to your logical mind). When your ex has sex with others (or even just makes out with others), it can thus feel as if he is cheating on you and deceiving you, because you never accepted that the relationship is over. Of course, this way of thinking presents a disconnect in your reasoning and in particular in the way your brain communicates with your heart. So, it is not logical, but it can feel heartbreaking nonetheless.
2. Jealousy often is not rational. Jealous behavior may be rational if you are at great risk of losing your partner to another person. But in most cases jealousy (despite its potential evolutionary benefits) is irrational. Jealousy in those cases involves a false sense of ownership. We often feel that we own our lovers and crushes. We feel that we are entitled to, or have a right to, their love, affection and advances, even when nothing of the sort was promised. Logically, we know we cannot own another person. At best, we can make agreements with them—they can promise us fidelity, but we do not own them. Jealousy can nonetheless grow out of a false sense of ownership. Just as we wouldn't like if someone borrowed our newest and dearest material possession without our permission, we don't like it when others take what we feel we own, in this case our exes. As illogical as it may seem, this may be a reason you are feeling so horrible about your ex's past escapades.
3. Even if you have absolutely no reason to think otherwise, it may be that you cannot help but wonder to what extent your ex is thinking about his past lovers while you are in bed with him or are being intimate with him. In some cases, the past lovers were the ones to end things with your ex, which raises the questions: does he ever wish the past lovers hadn't ended things with him? Does he still fantasize about them? Does he compare them to you? Was the sex with them better than sex with you? Was he more attracted to them? Irrational as it may be, these thoughts may be part of what is bothering you.
4. We are created by nature to be sexually competitive. While love is partially socially constructed, sexual attraction and competition are primarily evolutionarily based. Even if your guy is telling you that the others he slept with didn't mean anything, a part of you cannot help but think that he is comparing you to the others. The old evolutionary part of your brain still regards the past lovers as competitors. It is as if that part of your brain is thinking "if I fail in comparison, he will return to the competitors." There may, of course, be circumstances where this could happen. But in most cases these thoughts are illogical. If he chose you, and really chose you, he has no contact with his past lovers (or at the very least no interest in them). So, you ought to relax and go with the flow. Unfortunately, owing to the ancient areas of our brains, this is often easier said than done.
5. In some cases, there may still be a risk that the other woman constitutes a threat to your relationship. If your ex slept with people in your common social circles or people that inevitably will have to be part of his life (for instance, a co-parent of your child, a co-worker or someone he does business with), then these other women could in principle still constitute a potential threat to your relationship. They may be successful in seducing him in compromising circumstances (for example, at the annual Christmas party). So, the ancient part of your brain is in constant alert mode, making the very fact that he slept with these women a real obstacle to getting intimate with him again. This may or may not be a legitimate concern, but it is a concern that can be hard to shake

What to do? Even if your ex's past lovers were the ones to end things with your ex, there is a reason he is with you now and not others. If you are serious about each other, you should be able to talk about these things. Sure, your ex may not be forthcoming about how sexually advanced that chick was that he was sleeping with while you guys were broken up but remember not all guys are into sexually advanced chicks as long-term partners. As hard as it may be, if you were broken up with your ex at the time when he had sex with others, you are entitled to your feelings but you are not entitled to blame your ex or be mad at him for doing what he did. You could have done the same. Maybe you even did do the same, in which case you should be able to take some consolation in the fact that you are now equals in this regard.
Berit "Brit" Brogaard is a co-author of The Superhuman Mind and the author of On Romantic Love.