Can’t maintain erection, nor achieve orgasm.

Patient: Hello, I am an 18 year old male and have started having regular sex with my girlfriend a while ago.The problem is:1. I often lose my erection during vaginal intercourse.2. I find it incredibly hard to reach orgasm through sex(whether it be vaginal, oral, or with hands).Things to note:1. My girlfriend is attractive and capable in bed, so I’m pretty sure there is something wrong with me.2. The only times I can reach orgasm is after receiving at least 30 minutes of intense stimulation, all the while trying very hard mentally to bring myself to “come”.3. I have no problem getting an erection, but my penis starts to go limp when engaging in slower sex or vaginal sex.4. I can reach orgasm much faster while masturbating.5. I am not circumcised, my penis looks healthy to me, and it is 6 inches long with average girth.Could you please help me?P.S. I am thinking of trying Viagra.

Symptoms: Erection is weakened often during sex. Sometimes lost completely.
Cannot reach orgasm easily. Often have to give up.