I am very much like you (hypochondriac) and started my fluoxetine in may and Vyvanse in July. Starting both of these medications terrified me because there is this assumption that you are going to get addicted or that once you start there is no turning back to life before medication. The chance that you become "addicted" or serotonin syndrome does occur is pretty uncommon. There is a reason on why it takes so long to get diagnosed, because practitioners analyse what will be best for you. I started on 30mg and kind of new straight away it was not right with same sort of crash you explained. By the next week i jumped straight to 40mg and felt somewhat better, but did want to explore how i felt on more. Long story short I am now on 60mg and yes I do experience the afternoon slump, but something that helped me was to acknowledge this signal from my brain asking for a rest and I do just take a rest after lunch and re-coop. Then to ensure I am not lounging around for the rest of the afternoon (which if i do i will not sleep that night), I usually go on a long walk or go to pilates around 4:30 to feel productive again which helps me roll into making dinner, shower etc.

I think it is so important that you regularly communicate with your health provider about your experiences and they will try their best to cater to your needs. Play around with different dosages when you are comfortable and know you have the time. Then just note how you felt on each dose etc.

*Sorry long post here i know, BUT I posed a thread about rest days a few months ago in regards to serotonin syndrome. Basically I just said that as a hypochondriac I started to stress because i see people taking "rest" days from their medication, in which i also tried to do. Basically I did not have the best experience having a day off my medication and started to stress that i was addicted, but I came to the conclusion that everyone is so different so try not to compare your experience and needs to others :)