What should I do about this?

Ive been dating this girl for about 4 months. I really like her like alot. Great person the only problem is the .. I can only last 20-25 minutes with her. I've never had this problem with any other girl. She tries to re- ure me each time that it was fine but I know it's wasnt. What is a way I can last longer?? And no im not gong to , that is completely disgusting..