What's wrong with me?

So...i'm 42 and my problem is kinda...delicate. i can't...finish. it's been like this in most relationships over time, except one when i was around 20 y.o. and later around 36, with a toxic girl that made the sex amazing. Now...nothing. my current girlfriend is fairly annoyed by it cause she thinks it's her fault, even though it happened one in the 4 months we been together, and that's because i performed some manual labor...beforehand. Other than that, nothing. Now, I eat fairly healthy, i take supplements like zinc and some vitamins, i go to gym on occasion for leg days mostly, so i'm not a slouch. I do watch porn rarely, and when I do it works perfectly. I'm trying to avoid it altogether but sometimes i just need it. Like I said, it's rare. So...any input? Thanks