What Causes Erectile Dysfunction?

I am a 19 year old male that has recently ended a 2 year relationship. Sex was a large part of our relationship and was had frequently. The only times I had eperienced any sort of ED was when we first started having sex, this didn't last long and never happened again. I have moved on and recently became physical with another girl, yet my problem has come back. Twice I have been unable to get an erection, I am very attracted to this girl but nothing seems to help. I know that i am eperiencing phycological ED but have run out of patience and would like to know my options. Is there any product or anything you would recomend until I can overcome this problem?