Chronic diarrhea. Mild but leaves me feeling terrible. Anyone relate?

Will try to be brief! About 45 years ago, I picked up a stomach bug while abroad and my digestion has never returned to normal. The issue is diarrhoea, albeit relatively mild.
If I take 1 or x Loperimide per day, I'm generally OK, apart from occasional 'bouts' which my question refers to.

I should say first, despite countless investigations, nothing much has shown up apart from "mild to moderate bile acid malabsorption" but that was at least 10 years ago and I've recently had another test that proved negative for that

My problem usually happens when I eat too wheat (though various tests have never shown any allergy or coeliac issue).

To get to the point, what happens is that I have an uncomfortable urgency to empty my bowels. The stools are only a little bit loose but almost immediately, I feel absolutely terrible - it's difficult to describe but exhausted, a bit sick, a vile feeling in my head and throughout my entire digestive system. I have no energy and I just want to collapse.

Typically this thoroughly unpleasant feeling lasts about 48 hours. I don't have any further BMs (until one, the next day). I just don't understand why this happens and no doctor has ever been able to give me any kind of explanation.

How can one slightly loose BM make me feeling SO dreadful for SO long? If I can't get a resolution, I would at least like to understand the logic. Can it be that I'm simply dehydrated? But how can the sensations be so sudden and anyway, it's not like fully blown diarrhoea where there's repeated BMs and lots of lost liquid.

Most of the time it's fairly OK and manageable but it's often worse when I'm abroad, as I am right now (in Italy, where it'squite hard to avoid wheat). Final comment, before I discovered Loperimide about 25 years ago, my life was sheer misery. I had diarrhoea every day, multiple times, liquid. I felt so incredibly awful every day. At least it isn't like that any more. I don't believe anything will ever be "found". I think it's a dysfunction based on nervous system dysregulation, or something like that.

Can anyone else relate? Does anyone know how or why my body's reaction can be so very extreme or how a little 'touch' of diarrhoea can lay me low for days??? Sorry this is so long! Many thanks in advance.