im uncircumcized and vagely remember the pain i experienced when i was younger and i pulled my foreskin back but i do know that the more i pulled it back to clean it the easier it became and less painful to pull back.
i think the fact that i started masturbating when i was about 11 and have done on a constant basis also helped me out a lot.
as for your situation i would suggest trying to pull it back while you are still limp to help the skin get more used to being stretched (you should do that when you clean anyway) aswell as trying to slowly stretch it while you are masturbating.
on a plus side, you can get your girlfriend in on it so it is a little more interesting
