Why cant i come with my gf?

ive been with my gf for more than 7 months and we've had probably 15-20 times. i cant come with her but i can come by myself with some . im athletic and in great shape shes not athletic and shes a pretty girl but her body isnt necessarily smokin hot. idk wat to do. i love her with all my heart and i dont kno wat to do so i can come with her ive tried cuttin out masturbation but it still didnt work. i really love her but i dont know if its me or her or both please someone give me advice that will help us