If you get physically fatigued from holding your heart rate at 100, then even I, as an admitted not-a-doctor, believe you need to get yourself to a cardio specialist, and get a full workup, like... now.
From what I understand, carbohydrate intolerance is very similar to lactose intolerance, and both are equally (and easily) treatable/manageable. I genuinely can't see how this would not have been clinically diagnosed long ago.
The heart thing is something else entirely.
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