Perhaps you just don't fancy her enough, or maybe your nervous and thinking about it all too much. It's quite common. But yes I agree with Nohard. Give up the ****.
You are either some kind of nut if you dont know the answer? OK smoking, drugs and proberly drinking yes, lack of sleep, not eating properly, its your system its collapsing or did collapse, so the only thing is to give up smoking, the drugs the alcohol, get off fast food, start to eat healthy, get out and exercise, walk, jog, swimming, the gym and keep of sex for a while, its time to let your body heal and clean your system out, then perhaps after a few weeks of this then you can get back into action between the sheets, but take a big tip, give up the smoking and the drugs, the odd drink is ok but if you can just stick to beer and wine, hope this helps.
Good Luck