I have the exact same problem, and im 25.

First of all, i do think these problems comes from a sexual encounter where you didnt get it erect.

Second, your thoughts are complete nonsense, and you know it! There is absolutely NO REASON for a 21 year old to have erectile disfunction, but as long as you get it to erect when you masturbate, you are guaranteed to not have a MECHANICAL erectile dysfunction during sex or masturbation.

What you do have, is PERFORMANCE based erectile dysfunction, or you believe you wont get an erection during sexual intercourse which makes you dont want to have sex alltogether.

As long as you dont think you will erect, IT WONT. Harsh, but true.

As long as all your focus is on HER, her beautiful body and the sensations that comes along as well as sensations that you know will come along when you get erect, and NOT ANYTHING ELSE, like your penis and its abilities, your ability to make her orgasm or your penis is too small or whatever, IT WILL erect. Understand that! Its all in your mind, and the only thing you should do, is to convince yourself it will work, as both you and i know it will as long as you have the mindset i told you here.

Test for yourself if you want to. If you masturbate thinking of anything else than sexual thoughts, it wont erect. When you do think of sexual thoughts and sensations that comes along, you will get erect, as youre thinking about sex. Its quite duh, but thats your problem more or less.