Can’t live like this no more

Hey everyone,
I’m an 18-year-old guy and I just need to vent, but also desperately need some reality checks or success stories. To be honest, I feel like absolute pure shit and I'm losing my mind.
Looking back, before all this thyroid drama, I used to be energetic, highly motivated, and just fully healthy. Now? I feel like a total zombie. A month ago, I caught some nasty sinus infection, and since then, everything went from bad to unlivable.
Here is what I’m dealing with right now:
The Post-Workout Crash: Whenever I try to do a strength training session, it completely destroys me. I end up floored, feeling so sick and drained that I can't function or even hold a conversation.
Insane Brain Fog: I used to be highly motivated and driven. Now, I have trouble even visualizing things in my head or finding purpose in anything.
Morning Hell: If I don’t drink coffee first thing in the morning, I feel like I got hit by a train or like I’m experiencing the worst hangover of my life.
Medication: I've been on Euthyrox/Levothyroxine for about 6 or 7 weeks now.
My biggest questions for you guys:
How do I know if this is a massive Hashimoto's flare-up or post-viral fatigue (or both combined)?
How long does this post-viral/flare crap usually last?
Will the quality of life ever go back to how it was before the disease? Can I ever be that energetic, healthy guy again? I just wanna live like I was living before.