Welcome to the community!
Though I can’t rule out that you have an undiagnosed condition that’s making things difficult for you, I seriously doubt you’ve permanently messed up your brain due to your lack of study.
My suggestion would be to prioritize consistency over duration, and slowly work your way up. So, if I were you, I’d create a /study Beeminder goal, and set the slope to 5 minutes a day. Then I would do that every day until 5 minutes felt consistently manageable. Then I would up my commitment to 7 minutes, etc.
I think by staying consistent while reigning in your ambitions, you’ll see progress while avoiding burnout, and give your brain time to learn the new skills needed for studying. While studying challenging material will always be mentally taxing (and should be, in the same way working out in the gym should make you sweat), I think you’ll find that you’ll get used to the strain over time, and may even start to relish it.
In addition, I would also create some additional meta goals to help me improve my study habits, routines, and strategies. For instance, if I were in your shoes, I would create a /study-upgrades goal, starting with a modest slope of 1 or 2 upgrades per week, to which I’d add a +1 data point every time I did something that has the potential to improve my ability to study in the future. So, that would include things like installing a pomodoro app, organizing my desk, buying a desk lamp, or anything else that improves my ability to study. In this way, you can start to take advantage of the power of marginal gains.
One thing I’d look at for some of my first upgrades would be ways to reduce the friction and distractions surrounding my study time. Exercising self-control is a taxing thing in and of itself, so anything you can do to eliminate distractions during your study time has the potential to make a big impact. If Reddit is your vice, consider using an app like SelfControl, or studying away from your computer altogether. If you find yourself struggling to stay off your phone, put it in another room. Decide on one place where you always do your studying and nothing else, and keep that location clear of distractions so your brain knows what to do when you sit down.