Can't reach climax during intercourse

I'm a 29 year old man and I've been with my wife for nearly 14 years.  We've had a pretty active sex life for much of that time but I've been noticing my sex drive decline over the last few years. Nothing spectacular....What was 5-10 times a week when we were 15 became 3-4 at 20 and down to about once a week these days.

The big problem is that I've also noticed it has become increasingly difficult to climax during sex. It's been getting more noticeable for the last 3-4 years. Some nights (ok, most nights) it's so difficult as to be impossible, these days. It was an annoyance rather than a problem until recently when we decided we're ready to start having children, and my "incredible stamina" became an inability to impregnate my wife.

I don't have problems getting an erection, but after marathon sessions just trying to climax, when i'm exhausted and discouraged, maintaining it can be problematic. Usually I give up before that point (after only 45 minutes or an hour). I'm about 225 pounds right now, but I don't think my weight is the problem as I'm down from about 300 pounds 5-6 years ago. I almost always have one beer a night after work, I smoke pot a couple times a week, and I've been known to do LSD or ecstacy on the rare occasion I can get it (not in more than 6 months for both).  I do masturbate a couple times a week, and I don't have any problems then. I tried stopping for a couple months and it didn't fix the problem.  I may have failed once or twice.  Sue me.

Is it normal to be having these kinds of problems at 29?

I can't think of anything else to add here. I'll answer any questions I can.