. . . I've found myself highlighting all the good things about the other women and not feeling very good about myself. I'm thinking... "He's with me because...."
Well, you can always just ask him what he likes about you, why he sees you as compatible, how he feels when he's with you, etc.
No one ever wins at the compare game. There will always be someone else who has attributes we don't have, we wish we had, or whatever. So, while it's not a bad idea to take stock of your gifts and acknowledge what you've got going for you, don't give in to comparing. When you catch yourself following that train of thought, just stop it! (You know, like how Cesar Millan - the Dog Whisperer - diverts a dog's attention from a bad habit with a sharp noise or a nudge) Occupy yourself with other more practical activities, pay your bills, call a friend, go shopping, hit the museums and look at some art, exercise, whatever. Just don't give in to comparing, because it's always a losing proposition.
I think when someone comes into poly from a strongly monogamous mindset, we will tend to automatically think in terms of comparison. But the assumption that a partner will view some of his or her sweeties as better than the others, or that there's a "best," or that it's a competition is more mono than poly. People who want and can have multiple relationships see many different things they find appealing about various partners and can appreciate those differences. So, it's not about choosing a best one and setting others aside; it's about enjoying everyone for all their differences (or similarities!).