My penis size is medium but i can't put it in a girl vagina and she is not a virgin . why i can't stick it in ?

You have to preheat the oven before you stick in the turkey 

I used to very tight for my hubby too but got used to it. Try using lube at first it will help.

You know that you are required to have an erection before proceeding with penetration right? Once you have the little soldier at attention and ready for action, spit in your hand and rub it on your penetrator, like you usually do when you have sex with your self. Once it is wet and slippery all over, slowly slide that violator into the honey pot. She might squeal and wiggle a bit, but that's to be expected the first time. Keep slowly entering until you are up to your balls and can't get in any deeper. Then just wait a bit for her to get used to and accommodate her "guest". After a bit, slowly start moving a little in and out until you have that explosive gusher we all love. Then wait a bit and do it again. Now you  will have another gushing orgasm, and you will leave that little bearded clam much wetter than you found it.

Because you're a stud and don't know it.

Vaginal openings also vary in size.

ah you gotta take your pants off bud.