Erection problems

Patient: So i’m 17 and I have trouble with getting erections when it comes to sex. I used to be able to think about sex and i could get an erection but its become increasingly harder, and its kinda scaring me. I’m fit, not overweight and i don’t do anything besides hangout with friends, go to school and play football. i’ve been having trouble for almost 6 months. what should i do? or what can i do to make things go back to normal