There's a LOT that can cause fatigue... Your thyroid medication might need adjusting. You could have vitamind D deficiency. (I know I am deficient and can't take the supplements, so... Taking in as much sun as I can to fix it, but at the end of winter/beginning of summer I am knackered to the point of pain.) The diet is worth it, honest. It makes a huge difference to quality of life and the length of said life. You should really get a blood glucose meter to see whether your dietary changes are having the impact they should, so if it is high bloodsugars, you can suss out why they're high. What do you usually eat in a day? And also important: are you getting enough fats and protein? There's 3 macro nutrients: carbs, fats and protein. If you cut one, up the others. Protein can up blood glucose slightly, though not as much as carbs, and fats are a blood sugar flatline, so ideal. If you don't up the fat to compensate the loss of carbs, you get deficient on vitamins and minerals, which also can cause fatigue... So you really need to balance things a little, if you haven't already.
Just don't let it go. You need to know what is causing this.
Good luck,
Jo