Can't achieve erection while standing

I am 24 years old and I have only had sex 3 times, all 3 of which have been a struggle. Everyone tells me to calm down and just relax and I try the best I can but it doesnt work. I have difficulty even while masterbating to achieve an erection while standing or on my knees. When im sitting back or in a chair or lying down it's no problem but while standing it immediately goes away. I'm still in the mood but all fails. I was wondering if anyone else experiences this problem and what I could possibly do to correct it. I wake up almost every morning with morning wood so i know the problem is not physical ( at least I think ). Please let me know of anything I can do to fix this problem. Thanks