I'm really sorry to hear you are feeling so down.
I'm glad you've reached out here, I hope it helps you to get some of your feelings and thoughts out of your head, sometimes it can help to clarify things for ourselves and just to feel heard and validated.
It sounds like your breakup last year was a bit of a catalyst for feeling down and struggling with your schoolwork. Have you been able to talk to maybe the school counsellor about this? Or contacted kidshelpline or headspace (both these have online options if you prefer) ... I think one of these would be really good for you, sorry if you've already done this.
It can be really draining in friendships in which you feel you are the stronger one or the older, more responsible caretaker type ... but friendships should be a two-way thing. You may be surprised at your how much your friends would like to help you if you only let them? Vulnerability is hard, but there is love and care available to you from your friends. I wonder if you could try opening up a little bit to them and see what happens? What do you think.
You are very welcome here to talk things through.
You are not alone.