17 year old athletic healthy male, but last few months have been having trouble getting hard when with girlfriend (not everytime but enough for serious worry) I used to be very horny and it's causing lots of anxiety and stress in relationship?

Performance anxiety.  You are putting too much stress on yourself.  Stress and anxiety can cause impotence.  You are basically your own worst enemy. Try to relax, stop anticipating, enjoy each others company and let it happen naturally if its going to happen. 

Wow 17

Not even grown yet