I wouldn't worry mate. Hocky is right, my first time I didn't come (it was sex, not a bj but i think it still applies). i've had great sex with my partner ever since.
on bjs, i love them in porn and i'm turned on by the idea of it but i still take a long time to come that way. maybe i'll ask my girl to try it with my foreskin on next time. i'll let you know how it goes :D
but yeah, definately get a bit more experience under your belt before you get hung up on this