Hi, Welcome to HealthcareMagic Forum.
Do you have any past history of sexual trauma or abuse or psychological factors? You are just stressed out and it is causing Vaginismus
which is an involuntary spasm of the muscles surrounding the vagina. The spasms close the vagina which causes pain when he tries to penetrate you. Sometimes the cause is Unknown. Get yourself examined by a Gynecologist so that she can evaluate you completely and do a pelvic exam and rule out the exact cause.
First of all just try to relax and concentrate only on this thing while having sex. Do repeated pelvic floor muscle contraction
and relaxation (Kegel exercises). Try it more than thrice a week, If these measure are not helping, you should see your Doctor immediately.