Jealous of his past, what do I do?

Ok so I met my current boyfriend last year when he was still with his ex. Within the first three weeks of knowing him, he found out that she cheated on him (she lived in Jersey, we live in Alabama). I didn't know him very well at this point but I could still see how much she had hurt him. He broke up with her and then sulked for about a month and a half. Then he started to flirt with me until about January of this year. That's when we decided to get together. Well we've been together for five months now (longest relationship for me). Last month he moved about an hour away from me. I told him that I would help him clean out his bedroom at his sisters house (he'd been staying at his aunts house, hadn't lived at his sister's since the breakup, but had slept there a couple times). When we start cleaning out the room I noticed a pair of undies in the drawer and they were "dirty" ones. I pointed them out to him and he starts making references as to how the crouch of them got "dirty". I was disgusted and upset at the same time, so I walked out of the room. When I came back in a little while later he had thrown them away so I was fine. Until I got to a decorative box, I asked him if he wanted to keep it and he told me that his ex had made it and that he wanted to keep it for the memories it held. That also upset me but I just kept it to myself. Later I found a small pillow she had also made for him. Of course he had to keep it too. So when I finally asked him why he kept all these things from her after all the pain she had caused him, he answered, cause I loved her when she gave them to me and they remind me of the good times we had together. Now me personally, I like to destroy everything after a breakup, especially if it was a rough one. I asked all my friends and they all do that too. He also stores old pictures on his laptop of them from when they went to NY. He says he keeps them because he wants to remember the trip but I think its because he wants to remember her. I'm constantly comparing myself to her. I don't want to be her. She was very unattractive and nasty. I'm a clean and semi-descent person. I can't see though why after all the hurt she caused him, why he would want to still remember her. How does one become not jealous of anothers past? Is it possible? Or am I just crazy to think all of this? I fear one day that he'll send her something online and try to get back together with him. I don't know what to do? Any suggestions?