Sorry to hear of your struggles!
While I'm glad to hear your GP is trying to help, a little basic thyroid knowledge would help him a lot (he clearly doesn't have it). You could share a book like "The Complete Idiot's Guide to Thyroid Disease" with him so he would know the right tests to run (FreeT4 and FreeT3) to properly monitor your treatment.
He would also be aware of the fact that the AACE recommended TSH ranges similar to the ones your first lab used and how many labs haven't changed their ranges (as your second lab hasn't).
He would also learn that your TSH level really doesn't matter except that it should be 1.0 or lower if you are optimally medicated.
A quick look at your previous posts revealed the fact that your doctor had declared you hyper when your TSH went low. That isn't accurate. True hyperthyroidism only happens when the actual thyroid hormone levels - FreeT4 and FreeT3 - are over-range.
In addition, your doctor did not titrate your meds dose appropriately. That alone could be responsible for the adverse effects you had on 75mcg. It could simply have been a matter of too much too soon.
Unfortunately, you had to backtrack and start all over.
Doses should be increased in 12.5mcg increments and, very occasionally, in 25mcg increments. Slow and steady wins the thyroid disease race.
Recent studies have shown that the EBV is strongly associated with autoimmune diseases of all types and there is thought that it triggers Hashi's....even if Hashi's doesn't develop for many years later. It was also found that people with Hashi's tested positive more often for EBV than people without it.
If the EBV is at its most active stage, you'd be bedridden with mono. It sounds like you're "just" dealing with inappropriate thyroid hormone levels.
So, it's not that you're having trouble getting regulated, it's that you are not taking an appropriate dose of meds and/or might have been titrated inappropriately.
Hopefully, your current dose of 50mcg represents an increase from your prior dose.
Labs in 6 wks. after starting this dose will show just what 50mcg accomplished. More likely than not, your FreeT4 and FreeT3 levels will show room for improvement so, another dose increase will be in order.
The next dose size in line is 62.5mcg and you'd need to take that dose for 6 wks.
The meds titration process is a slow one. However, with labs every 6 wks. and appropriate dose adjustments, levels can usually be optimized within a year.
Your doctors' lack of knowledge has delayed your return to wellness. You are clearly hypothyroid and need meds in an appropriate dose.
Since your GP is willing to work with you, I suggest you empower yourself with knowledge and guide him.