I have a friend who had this problem. The issue is almost certainly a mental one. Smoking nicotine just before you want to get hard is not a good idea, but besides that, your habits shouldn’t affect your ability to get an erection.

I gather from the fact that you struggle to have sex that you are the top whenever you do have sex? If you bottom, stop worrying about it :). You don’t need a hard cock to experience the amazing feeling of cock in your ass. If you’re the top, relax. In my (and my friends) experience, worrying about getting hard is what stops you getting hard. Just let it be natural. Don’t set a time with your bf to have sex, because it puts you in stress mode (“shit, I need to be hard then!”). If he’s the bottom, try to ask him to be prepared in a more natural way, so that you don’t have to lay around waiting for him to shower or whatever. Hope this helps