@TheJungleFour - congrats on a great start, and glad to hear that things have gone well having stopped the Gliclazide - I'm assuming you haven't yet discussed that with your GP? It may be a good idea to just let them know. In the end everything is your choice, but always better to keep the medical team on side.
"I'm curious what it is about high blood sugar levels that actually makes you feel so tired?"
What a great question - it seems totally opposite to what you expect - have something sweet for a boost ...
Two simple things at play:
1. Insulin doesn't work to bring back blood glucose to "normal" - in the case of a rise and fall (trying not to use the "spike" word) the end result is usually that you end up with less blood glucose than you started with - so having something you expect to boost yourself will usually leave you with less available energy.
2. Your cells don't like too much glucose. Assume that your insulin is working perfectly, and shunting all the glucose from your blood into your cells - now your cells have more than they need - oversimplifying, but what happens when you overload a fire, it burns less well, and produces more smoke - same thing in your cells; you produce less energy, and more free radicals.
On top of this - your insulin is likely not to be working perfectly - and particularly in your brain; your brain needs to be sensitive to insulin, first to allow insulin inside the brain, then to allow insulin to tell the brains cells to burn glucose - both of those things are hampered if there is insulin resistance.