So I'm looking at either anxiety or poor circulation and maybe even just a refractory period between erections. If this happens more than rarely, you should have a doctor check it out. This could also be the cause of the numbness in your penis - the rigors of sex could throw your back out. Now, perhaps you're also suffering from one of the bad effects of pornography. And it's not a feminist plot.
Sex produces endorphins, which are opiate-like substances in the brain.
That's heady, and for some, it's addictive stuff. One woman's image no longer does it for you. This check-up would be to look for medical causes of the change in you. The cigarettes aren't good for you and over time can certainly lead to damage to the blood supply of the penis with decreased erections, but at 21, that's doubtful to be the cause. Squeezing your penis hard every night might be also causing some local trauma and swelling with resultant loss of sensation.