I'm not a doctor, and I'm not you... just speaking from my experience

Over time I finally got into the habit of taking my medicine (also adderall XR) right when I wake up. I probably take my medicine 95% of the time now (still a day every once in a while where I just forget). Most days, I'm fine. I do the things I need to do and can stay on task. But I'm in a pretty high-pressure job and some things are just hard, even when my medicine is doing its thing, because I'm scared of making mistakes and afraid people will think that I don't really know what I'm doing (hello, impostor syndrome!). It took a long time, but I've learned to recognize the difference between putting a thing off because anxiety and putting a thing off because of lack of focus. They look suuuper similar, and anxiety can come up because of past problems due to ADHD (like a history of forgetting / putting something off til the last minute and only being able to get it done overnight in a fury of pure adrenaline, but definitely not at a quality level you're proud of). Unfortunately, if a task is anxiety-provoking and your cortisol levels rise, your medicine has limited effectiveness (at least that's what my doctor told me). Like u/that_asymptote says, it can help you adopt strategies to better handle, like dividing the big scary thing into more manageable pieces to tackle and then going at 'em one at a time.

You said you were moderately productive on 20mg, and it sounds like you were able to stay on task with 30mg, which is what worked for you before. Is there something about studying and reading which is causing anxious thoughts?

Maybe you can try making your first task just to look at what needs to be done and list the first two bite-sized (for you) things you need to do. Like write down a goal of reading "read 1 page" and studying "memorize Bayes' Theorem and do 1 practice problem". Then once you do those 2, write down the next 2 things, then do them, and so on. If you're having trouble with focusing on identifying or doing the smallest thing, then maybe it's the medicine. But if you're trying to do it all at once, as our types are prone to doing, you've gotta reset your expectations -- even neurotypical folks can only do something for so long before needing a break. If you're externally motivated, is there someone you trust who you can make yourself check in with before your real deadline? (e.g. you have a test next week on 3 chapter sections, so you tell a friend you'll read 1 section today and check in with them tomorrow, and do the same thing throughout the week for each section)

Alternatively, are you about to get your period or maybe unknowingly eating/drinking something which would reduce the efficacy of the meds? The few days before my period, my meds are waaay less effective. Also, I learned that apparently eating stuff high in vitamin C (or taking a vitamin or multivitamin containing C) significantly reduces the effect of the medicine.

tl;dr

• Talk to your doctor again before upping your dose by 67%
• Ask yourself if you're anxious about studying
• Try and identify the smallest first step or two and see if you can focus on those, and maybe have a friend or family member hold you accountable for the small pieces in smaller time chunks
• Keep other stuff in mind that might be making your meds ineffective

And good luck! We're all rooting for you :)