Stress can cause issues with erections, and then when you have a weak erection or difficulty getting it up that compounds the stress-- so it can build on itself. You said you did fine when you had sex with her-- focus on the positive. And figure out what you can do about lowering your overall stress levels. The fact you don't masturbate though sounds like you may have a lower than normal sex drive- at your age most guys are at their highest level of sexuality and most 19 yr olds are masturbating once or twice a day due to high sex drives. If you don't exercise much, start walking or running, that helps to 1) reduce stress and 2) helps maximize your testosterone levels by toning your body and reducing body fat.

you likely watch p0rn.... a killer for sex