Sex is not fun, it's hard work for me?

I smoke cigarettes but work my *** off everyday at a physically challenging job. When I have sex with my girlfriend, I sweat buckets and after a while start breathing heavily. Afterwards I feel worn out, now it's gotten to the point where I have sexual anxiety and would rather masturbate sometimes. I feel like a 1989 Chevy Beretta running on empty and I'M MUSCULAR AND SKINNY! How do I wax with out feeling tired and out of breath and sweating like a pig? Also, after a while of humping, my Johnny Wilson starts to get soft from all the intense discomfort my muscles and lungs get from "exercising." Is this normal when you are just starting out? Should I stop smoking, people have told me smoking has nothing to do with it.