I have my own thoughts on this. Years before I restored my foreskin I tried (like all guys do, I suppose) to study up and figure out how to be a good lover. I learned to last as long as any woman could want, and to sustain through several of her orgasms, but by the time I was 35, it was getting ridiculous. I had been married for 7 years and in all that time my wife had never managed to give me an orgasm orally despite concerted effort, and intercourse could take like an hour. I could have a few partial ejaculations (she loved the slipperyness) but then I would go sort of numb and if I wanted to really finish at all she would need to break out the dirty talk and the stars would need to align. I only failed to finish once that I can recall, but getting me there took its toll on my wife and she would need several days off from the roughness of it.
Since restoring, I’m getting such exquisite sensation that I’ve had to get back into the practice of deliberately postponing my orgasm or having a partial ejaculation so I could last as long as she lilkes. It turns out she likes to climax one or two times and get on with the day or nod off to sleep.
So what I recommend to guys who want to last longer is to give her at least one orgasm orally before penetration so the pressure is off. Once you’re having intercourse, don’t try to give her an orgasm with your penis. Give her an orgasm by rubbing her clitoral hood with your fingers. Another thing you can try if orgasm feels like it’s comming on quicker than you like - assuming you can stop short of a climax - is just interupting the intercourse for more licking.
I can also recommend those cheep vibrating rings that are sold by the condoms these days. That will give her a buzz where she needs it while possibly not over-stimulating you.
You mentioned that when you first got together longevity wasn’t a problem. If you were a long time between relationships before her, it could be that your mental map of what leads up to orgasm was based on the way sex is when you are alone. Intercourse wasn’t what you were conditioned to climax from so it took a while each time for the awesomeness of it to overtake you. So now that you have a lot experience with her having intercourse, your mind can faithfully project where it’s headed and what’s so freaking hot about her and you’re half-way finished before you start.
That sounds like an insurmountable problem, but I once heard a radio therapist say “there’s no such thing as a premature ejaculation - orgasm will come when it’s inevitable. It’s only premature if one partner stops pleasing the other before he or she has had enough.” So again I say get into practice giving her orgasms other ways. Lick her until she climaxes, then penetrate her and if you finish quickly and she wants more you can lick her some more while massaging her with a penetrative vibrator until she’s done or you’re ready to do her again with your penis. Don’t stop until she’s had enough and there’s no such thing as premature.
Good Luck,