I think you should take a step back for a second and try to have an epiphany real quick. Let me try and give you more of an objective look at things:

You're able to calmly explain your entire situation and all your thoughts and feelings, which means you're very perceptive, which means your intellectually capable: but there's a problem. For some reason, for 3 years you've felt helplessly sad to the point of self harm. Even though you can rationally explain that you understand that you don't want to die, and that your reasons for suicide are stupid, and even explain your reasons for staying alive, you still feel inclined to suicide. You're unable to perform properly in school now, and are now unable to think of any other way out.

It's crucial to try and look at your position objectively like this, because it shows your conflict, and how you're contradicting yourself. Understanding and realizing how you contradict yourself, is the key to resolving that conflict. You were doing well in school, but as you became depressed you began to show poor results, which of course made your depression worse.

Depression is a vicious thought cycle, that effects every thought you have, and it requires consciously changing how you think to be able to overcome feeling like giving up all the time. This is probably why you've done worse in school, which is a reason for why you feel depressed. Blame depression for your problems, not yourself. You are not depression, you are depressed, depression wants to become you.

You should talk to a professional for the sake of experience, and understanding. You're very young, and there's a lot that you're misunderstanding simply because you haven't grown to see the truth yet. I can assure you that there is more to life than what you're taught curriculum should be, and I assure you that even this small relapse won't hurt you like you think it will.

You should try and understand that you come in three parts: what you feel, what you think, and what you know. These parts coincide and react to one another, so much so that sometimes we rely on our thoughts from our feelings and our feelings from our memories. It's crucial to control all of these aspects, and to differentiate between them, so that you understand how your thoughts are forming. Your thoughts are controlling you, making you feel this way. If you learn to control your thoughts, then you will do better in school, and in life. Your goal should be to find out how to control your thoughts before you give up, so that at least when you rationalize you can do it clearly, and not have so many contradictions and questions and fears. Bide your time, good things will come.