Thanks for your query,based on the facts that you have posted it appears that you have what is called as Phimosis
.which prevents penetration of vagina during sexual intercourse with your girl friend.
Please consult qualified General Surgeon
for clinical examination to confirm the diagnosis.
You need to get circumcised .
This is a simple surgery done under local anaesthesia as a out patient procedure and offers permanent cure so that you can have enjoyable sexual life for forever.