Why am i having trouble getting an erection when with a girl in bed if i am only 18? how do i fix this? i still get full normal erections randomly .

See Urologist: This is almost certainly on an emotional basis since you occas get spontaneous erections. A brief visit can probably lead to improvement or complete cure.

Anxiety: You are suffering from performance anxiety. Try to stay relaxed when you are with your girlfriend. Certain breathing/meditation/yoga techniques can help you with this problem.

Anxiety: Possibilities include performance anxiety and the wrong partner. It's reassuring that you are getting full normal erections randomly.