I've actually been having this issue myself, and after speaking with a few of my friends in the escorting business, have been trying out a few things.
My health is very good too; I'm fit, go to the gym 5-6 days a week, etc. Hormone levels are great at age of 24.
Despite what my doctor said too - that it's mental and psychological, I have to agree with what DIST is saying.
I will be very hard leaking buckets of pre-cum, and the moment 'pain' is involved, the boner dies.
I'm a small guy: 5' 5", Asian, 130Lbs. Living in the US, taking those dicks with this small body can be hard.. And I'm always super tight.
So apparently, I have to stretch myself out. It certainly has gotten a bit better, once I started learning how to properly prep myself to try to ease myself into it.
I discovered that being on top gives me more control on the pace, then learned to tease the top in the process so that it's not awkward.
I actually use a few butt-plugs to give my hole a pre-stretch the night before sex...
Apparently the goal is to have sex so regularly that I will be stretched out enough that I won't feel pain.
Not sure how to get there though.
Once tried sleeping with a butt-plug in. Horrible mistake. The lube was dried up by the morning obviously, and my hole was so tight and stressed that taking it out was one of the worst things I've had to do...
So. Apparently. It's about dilating your hole. There are sex toys out for that... I just lost my patience and said fuck it.