There's probably some other confounding factors at play here. I've combined amp and propylhexedrine together even and didn't feel any sort of residual cognitive deficit although the drug itself feels "dirty" to me if that means anything.
i have been thinking that i am bipolar and that doing uppers causes me to go into a hypomania (the next few days i feel fucking great after the drug's worn off) that then goes into a depression which makes me stupid
Most typical Amphetamines have a fairly long half-life, so you are experiencing normal results of light-moderate to moderate usage. Afterglow/normal for 2-3 days then below baseline Dopamine along with Serotonin & NE levels. Below baseline Dopamine/NE decreases Cognative abilities. The lower it gets the more depressed you get and cloudier your head gets.
A real rough estimate is multiply the half-life by 6 to get the "afterglow" and/or "normal" and/or "withdrawal" delay. Thus we typically begin most Opiate withdrawal roughly about 24 hours because typical used Opiates have 4-6 hour half-lives. Methamphetamine has an approx 9-12 hour HL so we get 2.5-3 days roughly. Now look at Cocaine's HL of under an hour and it becomes more apparent why the crash and "fiending" can be so intense within a few hours.
Comedown and physical/mental "withdrawl" are a function of many factors such as: length of time on substance, dosage schedual, rate at which you increase dosage to adjust for tolerance, sleep schedual, roi, nutrition, and so on... the first four are major determinants especially for the Amphetamine class. The more you "shock" your Neurotransmitters the worse the depression. More you tweek and don't sleep the more intense the exhaustion. Cause and effect.
So a light user who keeps a decent sleep schedual will usually "feel" an afterglow for 2-3 days and function fine, then experience a week or so of Fatigue and mild depression. A light-moderate to moderate user may feel a mild afterglow
With some degree of fatigue for the 3 or so days after then likely significant fatigue with stronger depression.