I am a non smoker and i don't do drugs...i play sports. when I make out with girlfriend, i get a good erection, but when it comes to the penetration , i become very stressed that i lose my erection and can't penetrate.
I know from experience that it is easier to tell yourself to relax, than to actually do so! However, with some patience on your part, and your partners, I'm sure you will get passed this.
If the issue does persist, may I suggest that you post a question to the "Erection-Concerns" board? I'm sure that Dr. Epp would have some good advice for you.