As soon as my emotions hit me, all logic flys out the window. when she is with her other partner, I feel lost, insecure, self-conscious and I take it personal.
I'd suggest that when those feelings hit, you start asking yourself, "Why?" If you feel lost, ask yourself why. When you answer that, ask yourself why you answered the way you did. Keep asking why until you get to an answer that you can't ask 'why' to, and then examine that answer for clues to the real reasons behind your feelings.
I would also suggest that you be careful when throwing the jealousy label around, since I'm not necessarily seeing it here. Insecurities, absolutely, but not jealously. Do you feel hostile towards her partner, or do you fear that you will lose her to this other partner? (I would think not, since he was there before you, so if she wanted him over you then there would be no relationship with her right now.) You may feel envy that he gets to spend time with her, but ask yourself if you really are jealous, or if it is other things. Being able to figure out how you truly feel is important.
A big problem for mono/hetero guys (not typically faced by mono/hetero women) is that most men are genetically conditioned to want to secure a woman for themselves. This happens on an unconscious level, and is tied to a man's self-image of their own masculinity. If they are unable to secure a partner, they feel like they have failed at being a man. For women, on a genetic level, having more partners means more stability and security. Their main concern is more that their man is faithful to them.
In your case, you're probably wondering why you're "not enough" for her, right? You may feel like you are somehow defective because she needs to see other men. In your mind, you feel this reflects on you, since you inherently think that--in your mono mind--you should be able to secure a 'full relationship' with her, and not the 'partial relationship' you have now.
There are no easy ways to deal with those kinds of thoughts though. If you were a woman, I'd suggest going to find another male partner yourself. But the reality is that even if you wanted to find another woman for a partner, it's exponentially harder for men to find poly women than the opposite. Most women typically want a man that is devoted to them. Heck, it's possible this woman you are seeing may not want partners who aren't mono. Ask her about it, see what she says.
Personally I feel the only way to truly understand the poly mind is to become poly. If you want to understand why she is the way she is, then you need to become poly yourself. You need to be with a second partner, and still feel the same way towards the first. Once that happens, you'll have an 'a-ha' moment and begin to understand things better. In the meantime, I would strongly suggest you spend the time you don't spend with her, looking for another partner. Even the act of looking may help you understand the duality of the situation you are faced with.
Staying mono while she is poly is hard. REALLY hard. Usually those who have success with this were a mono couple to begin with, and then one wants to go poly. Since they have an established bond and love between them, it's not as hard as those who are trying to forge that bond when multiple partners are involved.
I agree with those above as well. You need to do things. Anything. Get out with friends, or get online and do some gaming or whatever. But sitting at home while she is out with her other partner is just going to be torture for you. The sooner you can break that cycle the better.