The vagina walls are made of muscles that can contract and relax in order to adjust to the dimensions of the penis. Both can be affected by physical sensation and emotions.
You might want to try some lubricants to decrease the friction which causes the pain. Also, you might ask your partner to penetrate extremely slowly so you can feel only part of the pain and to give the vaginal muscles enough time to relax themselves.
Using some relaxation techniques (yoga, for example) before intercourse might be helpful to decrease the tension you feel and keeps you from relaxing the vaginal muscles.