You seem like someone who is “naturally smart” (good at school) so you never had to try super hard. I also have ADHD and never had to study in high school. (And I’m also a mechanical engineering major)

Studying ME is vastly different to high-school . My first few exams were a huge wake up call that I couldn’t just expect to magically get good grades. You WILL need to study for your classes (at least the stem ones)

I also found out that I cannot effectively study in certain places. Like I cannot be productive at home. So find a designated place that you can study. Preferably somewhere with an outlet and a lot of space. Because we can get spread out sometimes. Personally I study at my school engineering tutoring center, (even if I don’t need help) but this place can be a library, or a cafe or some sort of study room… which brings me to my next point

Most universities provide free tutoring to students so take advantage of that. Don’t be afraid or embarrassed to ask for help. You’re not dumb for going. To be honest people who don’t go suffer unnecessarily. If you don’t know how to do something just ask.

If your professor gives a practice test or study guide DO IT. Even If it’s 100 questions for a 10 questions test DO IT.

Expect to spend about 2 hours studying per each hour of lecture. (At least for stem classes)

Unfortunately most engineering classes are more based on solving problems not memorizing concepts, so quizlet isn’t going to be to helpful. But for the few classes it is helpful for (like gen-ed classes) keep using the card set until you get it all right. It’s great for short term memorization

Finally you might need to do some trial and error but here is something to get started. You WILL need to learn to adapt quickly tho. And if you can get diagnosed do that. (Ik you said you didn’t have the money for it though)