You have too much thyroid hormone in your system because of the exogenous hormone (T3/Cytomel) you've been taking. In an attempt to lower the amounts, your thyroid has throttled back on the amount of T4 it is secreting. This lowered amount of T4, together with your now-skewed ratio of T4 to T3 in your body, is causing your symptoms of lethargy and fatigue.
Your body was not built to run on only T3. It wants mostly T4, which it will convert to T3 as needed. When you run shitload of T3, you upset the balance of the hormone in the body. Organs and systems that were designed to run by converting T4 to T3 (like, say, CELLS) are now malfunctioning because your body no longer has enough T4 in it. Simply taking more T3 will not help because that's not what you need.
Some people can run high doses of T3 and be ok; their bodies can tolerate it, or they just push through the side effects. Your body apparently can't tolerate it. That's just how you are. For example, if I take ANY T3, I get exactly what you are describing, along wiht depression. That's just how I'm built, so I can't take it because I can't tolerate those side effects.
Believe me, don't believe me, I couldn't care less. What I'm saying is true, and if you don't believe me or don't care, that's fine. It's your body and you're the one who has to deal with the consequences.