I agree that trying Vyvanse, or also Focalin, would be a more practical solution--Ritalin, to me, is just the nastiest stimulant around, and I know those repeated comedowns build up over the course of the day. Most people have less of that with Focalin or even Straterra (which is less of a rollercoaster than dosing Ritalin multiple times a day), though we all respond differently... Your mostly favorable response to Ritalin does suggests a favorable response to Straterra and Focalin as well, though.
You're in school, so it's extremely important you take only a minimal amount of a benzo, preferably a short-acting one that will wear off around the time your final Ritalin of the day does) if you do so choose to manage this drug-related problem with... more drugs. A mere fraction of your lowest recreational dose is what I'm talking about--your friend asks you if you're high 30 minutes after taking it, and you say "can't feel shit, must be counterfeit pills pressed in a garage somewhere in Ecuador." Benzos won't get rid of stimulation much in moderation, but they will cloud your mind and impair memory which you really can't afford when Ritalin is already necessary to help you do your best. I would take a minimal dose (look up Xanax pill strengths on Google as I don't want to type the specific amount I have in mind--once you see it and after you laugh, that's right, I meant very minimal indeed

) of alprazolam (Xanax) just before you typically start to feel shitty. You won't be able to point to its direct effects or feel at all intoxicated, but you should feel a little better, less edgy with lessened anxiety, at least. Give it a few days. Not helping but not hurting either? Double the benzo dose for the time being, fail an ADHD exam if you haven't already, and visit your doctor, proof in hand, for a
real solution. Don't tell him what to give you, just say the Ritalin works well but the side effects make you feel like crap, especially late in the day. That's his cue to say "Ah! Straterra!" and if it's not any better after a couple weeks, the average doc has at LEAST two other solutions to try on you regardless of how old you may be and possibly even five depending on how liberal he is with the script pad and how much he trusts you.
And I'm sorry, but Desoxyn isn't one of them
Not directed at you, but it had to be said: Speedball = (Heroin or Morphine) + Cocaine + syringe + vein, nothing else.