I am a 44 year old healthy male
You have bigger balls than a stallion writing that post on here - Kudos.
At your age it is unlikely to be anything medical and more likely to be some sort of mental block.
In the late 70's, early 80's I had a mate ( serving soldier ) who was known as '' Duracell '' because he went for hours - He was around 22 / 23 at the time ( lost touch over the years )
Probably nothing to worry about ( and you are most likely worrying about it, or you wouldn't have made the post in the 1st place )
Relax and go with the flow.