When you are struggling with depression, or another mental illness it can be incredibly hard to study. Hard to focus on your books or notes or hard to concentrate and remember while sitting in class,not to mention the lack of motivation when it comes to actually starting an assignment or getting the work done. If you combine anxiety and depression, where you have no energy or motivation to do anything with perfectionism where you want everything to be perfect. The smallest details have to be perfect, but never feeling like you've done enough or its good enough.... it's an awful combination and that's what I deal with at times. Not wanting to study or work at all but at the same time spending hours each day studying and still not feeling like it's enough, it takes a huge mental toll along with other things that evoke anxiety and guilt and the extreme tiredness.
When you struggle with a mental illness, you have to be kind to yourself. Sometimes that means taking a nap after 30 minutes of studying and sometimes that means forcing yourself to just do an assignment because in the end it's better to just get it don't so that you don't suffer the anxiety of a looming deadline or unfinished assignment.
Also talking to teachers and getting support. If you have a diagnoses it's easier to get some extra time with assignments or extra help from teachers, but whether you have a diagnoses or not you should talk with a teacher or councillor to get help or so that they understand. They are only humans and you won't be the only one who might need extra time for assignments or need to explain why you aren't in school some days.
Also set up plans and organise. Don't keep saying "tomorrow " because that tomorrow never really happens. Instead make a plan and decide to may do 30 minutes of work each day. Maybe read over your notes each day or read a few pages. Also do the most important thing /closest deadline first. I have a habit of first doing the small unnecessary things for just to get them out of the way even if I have 8 weeks to get them done, I hate having an assignment looming over my shoulder. But the best thing is to work on the assignment with the closest deadline.
Ask for help. If you need extra help or don't understand or can't get started with an assignment, ask for help.
Take breaks and fuel yourself properly. It's impossible to study if your brain doesn't have the fuel it needs. So eat enough and drink enough water. Don't just rely on caffeine and sugar. And take breaks and rest and get enough sleep. But don't take so many breaks that you never get work done because in the long term that won't help you.
You don't need to get top grades, but my personal opinion is that either you decide that your mental health comes first and you can't study and recover at the same time, so you take a study break. Or you decide to put you work in AND stay healthy. I.e don't force yourself to go through school if your mental health is declining all the time. Either you can balance the two or you can't. And there is no shame in needing a study break. Taking a break or working can be very beneficial.
At the moment I am definitely feeling that studying a little harder. Less motivation, my thoughts are on other things and much more tiredness. So I am doing the best I can. Studying a little everyday. Looking at my notes, studying with others and just doing my best. Not going to stress myself over top grades or being perfect in school. Instead taking the breaks I need and also writing down my thoughts that interfere when trying to concentrate on my school notes.
Be kind to yourself and don't overdo yourself. Hard work does pay off, but if you lose your mental health on the way it's not really worth it.