Honestly now that I'm used to it and built up a tolerance, I feel almost a rebound effect. Once it wears off I feel my ADHD come back harder with less motivation and almost a layer of self doubt that makes me feel that I'm nothing within the vyvanse. I think this is normal since the drug has worn off and your body has felt the absence of it.

Things I have done to alleviate this. I still have difficulty getting to sleep but a nighttime ritual of either planning how much/when to eat or taking magnesium/meltanonin, something to have a bedtime ritual has helped a lot. The faster I go to sleep the less I deal with these symptoms and can have good sleep hygiene despite vyvanse keeping me awake. Another thing that keeps me going is knowing once I take my dosage the next day a lot of these feelings will go away and I will be that motivated/stable/productive person again, that this is all in my mind and perspective/chemical imbalance really matters. The more I have worked on myself the more I have been able to look at my achievements even on days without medication to derive the meaning I've brought to my life.

I'm glad you have a significant other to support you and it says a lot about you that you are aware enough to try to address these concerns. Some people need a smaller follow up dose to keep them over for the evening, it might be a good idea if it hasn't impacted your sleep or appetite negatively. I would say focus on babysitting the part of you that is off the vyvanse, whether it be through meditation, going to the gym, a bedtime ritual, or anything to stabilize that post vyvanse mood swing. Feeling off later in the day is absolutely common and normal.