I totally understand your frustration.
I gained 20+ lbs after starting levothyroxine, & my BGs went to crap. Both are common. Levo causes insulin resistance. No matter how much I exercised or how much I restricted calories (I’ve been low carb for years), I kept slowly gaining. In May my PCP finally tested my T3, & switched me from levo to NDT (natural dessicated thyroid). She also started me on low dose Naltrexone. My BGs began to level out again about 6 weeks after stopping the Levo, & I have now lost 19lbs.
In my case, selenium did nothing to help me convert. My T4 was high, but my T3 was extremely low. I’m due for labs in the next few weeks. I expect to see a huge improvement because I’ve definitely had major improvement in my symptoms.
If you haven’t had a full thyroid workup, that’s where I would start.