Erection Probelm at 18

Hi guys, i really need your asvice. Iam 18 years old, since Iam 15 iave been gaving a hardcore erecions. It was enough for me just to think of something erotic and my erection was there, hard and long lasting (couldnt hide it) I was masturbating sometimes couple times a day, almost everyday. Now Iam 18 everything is normal and suddenly i meet this girl whom iam crazy baout, first week we date my erections are strong, soo strong that iam ejeculating in my pence. After 2 weeks of dating we decide to have sex and I notice that iam not having my strong erection for some reason, I felt shame, all i could do is tell her that i just ejeculted fast again (but really i just didnt get it) . Ive been to a horrible long depression for a week, almost cried a some point for being soo bad =( I talked to her about it and everythign was cool. Week later i meet her again, depression is mostly gone, we are kissing touching each other my erection is hard, again like first time i meet her. later at night we get to the sex, but iam exausted from the touchings and kisses, my penise is up but not solid enough to be able to penetrate or put condom properly (I can Tell). Since the second attempt another week passed. I feel like i dont have a desire, i cant stimulate my self with my mind. When I masturbate it takes me awhile to get it solid (almost masturbating without having my dickk 100% erected) Iam stressed out, i dont know what to do, I met my girl yesterday we were kissing and i dont feel as erected as before... wuts wrong with me? I do sprots, run, work out, eat Centrum vitamins , drink jices n stuff... it became hard for me to get erection, wuts the problem, iam sure its not physical its prolly mental... give me an advice please or tell me what should I drink in order to get a better desire and erection,
Thank you so much for our patience to read this story, but I really REALLY need your help