Heh, it doesn't sound shallow at all. My last ex was the best looking girl I've ever been with and it does cross my mind that maybe the girls in my future might not be as good looking. But you are going about it the wrong way, and you should have a more positive attitude. Easier said than done, and I think you have a little work to do on yourself before you are ready to date again. Which sounds horrible, considering how lonely (and dare I say desperate?) you probably feel right now. But trust me on this when I tell you that everything will be okay and you will find somebody else. And maybe, when that happens, her looks may not matter as much if you really get to know her and find out her personality blows your ex away. You should be looking for a PAL - Personality, Attitude, Looks in that order. Or maybe you will find somebody better looking. But you will never find anybody with your current attitude and feeling of self worthlessness.
Your self worth is not held by everybody else's opinion. I know you feel like a reject and that you aren't worthy of her after your break up, but you really are. You are a good person and your ex wouldn't have fell in love with you in the first place if you didn't have some great qualities. True that you weren't perfect, and you made some mistakes in your relationship, and I'm sure she did too. Looking back at your relationship, you can see some areas where maybe you weren't as good as you could have been (communication, treatment of her, honesty, etc.). This is your opportunity to improve upon that and become a better person and boyfriend for whoever is lucky enough to have you next.
3 months is not a long time man and it does get better even though we all hit the ups and downs. I'm on nine months myself and I'd be lying if I said I didn't feel lonely now and then and I'd be lying if I said I still didn't love my ex. But to me, moving on is accepting that you don't need them in your life to be happy and when you do accept that and accept the fact that there isn't really much you can do to change the past, you are taking a huge burden off your shoulders. This is your time to do what you want, and if you want to be a better person and boyfriend, start today. You have a job or school? Focus on doing better at that for high grades and promotions. You work out? If not, start to so you can build up some confidence and be healthy. What makes you happy? Do just that and don't worry about trying to get somebody else in your life. I'm guessing you are still young and the world is far from over. And the funny thing about not looking for somebody is when you aren't, they always come looking for you.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.