Not to undermind how you feel, but your situation sounds like no big deal to me. First of all, most people are asymetrical. My nostrils are slightly different shaped for example. Other people have different shaped/sized ears. Even different breast sizes are very common. I would say that at least half of the women that I've dated in my life have had a noticable difference in cup size between the two breasts. I'll admit that I could be picky about looks, but that didn't bother me a bit.
Keep in mind that people see you as you portray yourself. If you exude confidence, you will be more attractive to others. When I was younger I had no self-esteem. One day, a friend's girlfriend noticed me walking with a confident look about me. She brought it to my attention and told me that it was very sexy (she wasn't hitting on me, just telling me as a friend). I took that to heart and decided to turn over a new leaf. At first I had to pretend that I was confident, but as women started noticing me and I started getting attention, I didn't have to pretend anymore. I went from feeling worthless to being put up on a pedestal. Confidence is a powerful thing.
Another thing that most people don't realize is that handicaps and deformities can be a blessing in their own way. I'm not at all suggesting that you fit the description as I think your situation is way more common than you think. I'm merely trying to ease your worries about your boyfriend. The world is full of shallow and downright ugly (on the inside) people. If someone can see past your short commings and still want to be with you, you know that they truely love you for who you are.
Cheer up hun! I'm not just giving you some "feel good" fluff. I really think that your situation is no big deal. I believe that your hang-ups over it will bring you down more than anything. Realize that there's something that your boyfriend sees in you besides the breasts that he hasn't seen. Start looking at it as if he's LUCKY to be with you. We all have minor imperfections and if he can't see past yours, then he's NOT WORTHY of you. I'd be willing to put money on it that it won't make a difference to him though.