Help getting over Sexual Performance Anxiety..?

I'm only 17 so and i have no medical problems that make me unable to get erect, i can do it just fine when im by myself. I had one incident a long time ago where i couldn't get hard, and ever since then ive gotten so worried about getting hard every time that i have a sexual encounter, that i end up losing my erection. I usually have an erection during foreplay, but as soon as we go to have sex i start worrying about maintaining my erection so much that i lose it because im thinking too much. I need help turning my brain off and just letting my sub conscious take over. Any tips or advice?