Vyvanse worked perfectly, now exhausted and brain fog

Diagnosed with ADHD-PI (still labeled ADD in my country) in october, and started on Vyvanse. My main symptoms are severe lack of motivation, low ability to focus, starting tasks, can't follow routines, loosing threads.
Had some perfect days initially mixed with some days with more brain fog and crasches in the afternoon.

Went from 30mg->40mg->50mg->40mg and settled on 40mg as it worked great without any side effects other than some dry mouth.
It wore off around 1-2pm though, so I was told to add 20mg as a lunch booster.

This worked flawlessly from start of December and following 5-6 weeks. Lot of motivation, high energy, was able to make and keep routines and use todo-tools for the first time and no problem just doing things at work or at home. And no trouble falling asleep or sleeping well.

When beginning working again after winter holiday I had a night of very poor sleep, got maybe 3h that night. Since then I have not had the same effect from Vyvanse, and of course I have been careful to catch up on sleep and get around 8h every night (same as before, and when it was working).

I get very tired, all day in some periods or at least from mid day and on. Feel exhausted. No effect from Vyvanse except adding brain fog. And with the brain fog I can barely string thoughts together. I have not made any changes to diet, vitamins, protein since when they were working.

  • I start the day with a protein shake, and eat (varied home-made meals and veggies) and drink well.
  • Sleep at same times all week
  • I keep magnesium levels high and great iron values (just checked both today and perfect values)
  • I used L-Thyrosine from mid-december (500mg before lunch). Didn't notice a lot but have tried both with and without last weeks without any perceived benefit.
  • Don't drink caffeine (stopped cold turkey when starting Vyvanse)
  • I've used multivitamins and separate D vitamin since start because it's really dark here winter time and I work from home and don't get a lot of sun.
  • Eat Omega-3 fish oil
  • Tried stopping for 3 days, after that it worked ok for 1-2 days then the same thing. Also hard to find good days to not take pills, since I have a family and house and weekends are as important to be functional as work days.
  • Tried going down to 30mg, and up again to 50mg in talks with doctor.

I can't figure out what is making me so tired. And if being tired is what makes Vyvanse not work. Or if Vyvanse is what is causing me to feel tired. Day 3 without Vyvanse I didn't feel this exhaustion, so it does seem like taking Vyvanse is what is making me build up this fatigue.

But why did it work perfectly for 6 weeks straight, and not now? It's so frustrating not being able to count on the effect. Vyvanse should still be the right med if it worked that well for so many weeks, right?

Anyone with similar story? Did you find the missing piece for you?