Why do i feel so horny and my girlfriend is strocking my penis and i feel like cumming but my penis is not hard enough?

Look for cause first: To assess if one's erections have weakened, he should visit a primary doctor, who can check for possible causes: stress, depression, diabetes, high blood pressure, low testosterone, artery disease, high cholesterol, smoking, alcohol, etc... Causes found can be treated to help make erections stronger, firmer, longer lasting. A urologist can be helpful. Meds like viagra, cialis, or Levitra (vardenafil) can work.