I agree, it's very, very frustrating. But you know, everyone gets hard knocks in life. I would advise you to try and see that just because they dont have birthmarks, doesnt mean your peers dont have their own problems. I have lots of hemangiomas and therefore find it quite annoying when my sister complains about her one small mole! But I try and sympathise with her, because just coz I have hemangiomas, that doesnt make that mole any less upsetting for her.
And remember, this might be tough, but at the end of the day it'll make you a stronger person. Difference is beauty, flaws are character.