Most likely dry vagina.Did you try using Lubricants? Try KY Jelly.Have adequate foreplay before actual insertion.with well lubricated vagina and penis try inserting penis slowly all by yourself from woman above position
If the hymen
is very thick the doctor can open it by a small incision.
If you go into spasm of muscles,need counseling and reassurance .
As a last resort there are surgical operations to relax a tight vaginal opening.You have a treatable problem and dont hesitate to approach a Gynecologist.