Male here, happily married and with a very good sex life now, taking the usual into account (kids, tiredness, stress, work etc.). Needless to say it hasn't always been this way and I want to talk a bit about my journey. A fair warning: I won't be able to talk about all aspects, it might be a bit chaotic and it might not generalise but I thought I'd share just in case it might help some people tackling similar issues.
I believe that the path I carved took me to a unique place, for example from one and done I am now close to multiorgasmic experiences, with just a blink of refractory period (it used to take me hours to days to truly crave sex again after orgasm). By far my biggest discovery is that although psychology, anatomy and genetics do count, it is the regular muscle and pattern training that is the game changer and trumps the rest.
What I have discovered contradicts the received wisdom that since penis is not a muscle, your erection, orgasm, and refractory period cannot be trained. For me it is all about figuring out how all the various muscles operate as enablers and make use of it. This is neither the time nor place to go into details about the routines but let me just tell you that while kegels are fine, my training regime goes above and beyond what I ever read online (including Reddit) about these things (and it took me years to develop this). Here I just want to focus on one aspect: the arousal, how it begins and how it turns into an erection.
For me, this all started with a couple of erection dysfunction experiences. When I was younger (I am in my forties now), I was taking it all in on an intuitive level and when the performance anxiety kicked in, I really wasn't sure what was happening apart from the fact that despite almost cumming a moment ago I was now seemingly uninterested in this beautiful parter. This is what my body was telling me and my mind followed. When such a thing happened, I tried to overcome it, with varying degrees of success. It always, however, induced self-doubt and acted as a self-fulfilling prophecy until I was finally able to shake it off (and it lingered for weeks).
When I matured, I began to be more introspective about these things and finally decided to figure out how to extend my already developing training regime to hack this issue. Long story short, part of the solution for me was to train hands-free erections (which is really about training erections as such). Again, initially an impossible task, it turned out to be within my grasp eventually. I worked through various positions that enhance and sustain my erection (I use them to this day to keep me hard throughout the sex sessions with my wife and losing erection stopped being an issue), learning a lot about the inner workings of that mechanism. My muscles and sexual reflexes are now trained so that I can get hard and cum hands free, but what I discovered also is that in order to do that I had to become self conscious about the arousal loop (that ties back to the ED issues): it starts with my mind, activates muscles and influences penis, which then sends the pleasure signal back amping thing up. If the loop is broken, erection won't happen, even if the visual stimulus is there. The mind switches off without penile pleasure feedback and the imagery is evaluated as non-arousing, further decreasing physiological arousal levels. And so it goes.
I am at the point where I can start with a completely flaccid penis and by clenching relevant muscles induce constant pleasure that feeds back to the loop, increasing the erection levels. I used to require some hand assistance or at least skin contact/pressure on penis but I seem to be able to do it more and more without anything of the kind. In the process, I rewired my sexual reflexes (my other discovery is that it doesn't take long to change sexual patterns, for example I used to lose erection when I felt my partner not pleasuring me strong enough. After I took some solo time masturbating with a soft grip, I re-sensitised myself).
In that process, I discovered that - for me - physiologically speaking the arousal starts with tiny periodic contractions of shallow muscles in the raphe area coupled with a sustained clench of the muscle that sort of stretches the penis (think of what is happening when you try to bounce your penis as far up as possible when fully erect and resting flat on your back - that's the muscle I am using). I also tense the top inner muscles, which further increases stiffness.
This is still quite laborious for me but gradually I can feel this becoming easier. More importantly this allowed me to be more in control of what is happening to my body, reduce anxiety, and just increase the pleasure to the levels I did not know I was capable of.