Unable to maintain an erection

Im 18, and have loose foreskin phimosis, i have regular sex with my girlfriend, however on some occassions i struggle to even gain an erection or keep it for very long when i do, although sometimes i can easily and have normal sex.
I've been told i have a low blood pressure anyway and have thought about it possibly being the problem. Any ideas ?
Also, is there any way (not focusing on blood pressure) which could help me increase blood flow etc.
thanks, all ideas appreciated.

Hi Matthew - please don't worry. Do you get a normal erection when you masturbate? If so, then everything is normal. You may just be anxious - and worrying can cause these problems. Agree with your gf to massage each other sexually, with the purpose of enjoying being touched and also agreeing not to have sex no matter how hard you become. This way you will be more relaxed and can get to know each others bodies well without being worried about performance. When the pressure is off your erection may improve. Hope this helps, John