Could it be that you've got some deep rooted psychological reaction that fires off as soon as you start to reach any intense levels pain/enjoyment? Some subconscious trauma that gets brought to the surface? Basically that it is two conflicting emotional systems at play? I will freely admit that this is amateur psychology hour as far as I'm concerned, but I'll offer some advice anyway, (at least my psychological advice is cheap). Maybe all that is needed is lots of training in a safe environment so that the feeling of pain is not attached to anything horrible and traumatic any longer. If it's even possible.
That brings to mind a similar condition women often suffer, (forget the name?) which is that they tense up during sex which makes vaginal intercourse un-enjoyable, no matter how horny they are. This is an automatic reaction and is solved by them feeling loved and respected by the person they're with. They need to be with somebody they can relax completely around, for them to at all be able to enjoy sex. Could be something like that?
My motto is to never let myself be held back by any minute hang-ups I have. Breaking through the mental barrier no matter how terrifying usually has it's own reward. But that's just me, and I'm not sure my example in life is a good one to follow![]()