Welcome to the forums, congratulations for writing your first post and also for reaching out when you are feeling so sad, I am so proud of you. There are so many other young adults like yourself feeling very much the same so here not only will you realize you are not alone but you can share and draw strength from others.
I can hear that you are struggling with feeling purpose and feeling as if things are just too much and you don't want to stay, firstly I need you to know that there is hope Aimee and this is a time in your life, a time when you are not feeling well and you need some support, this does not mean the rest of your life and it certainly should not cost you your life.
There are some amazing support lines for young people out there, I cannot say loud enough how wonderful the people are at Kids Helpline, they are on 1800 55 1800, please consider a call to them as they will be able to give you some great advice.
I also want to share something from a parent point of view, see we do sometimes get it wrong and we do make mistakes, we are human too. I think if your mum does have some mental health challenges of her own it would be upsetting for her to realize that her daughter is struggling too, however, please do not stop talking to her, if it is easier perhaps write her a note, let her know exactly how you feel, be honest. It is not easy being a parent and you do feel guilt and shame when you get it wrong, she might be feeling some of these things too knowing you are hurting yourself.
Distraction will be your friend at times when you are feeling so very bad and like you do want to harm yourself. Please try to put even a list together that you can refer to when you are feeling like everything is too much.
I hear you when you say you are finding it hard to find things that make you happy, what were the things you did used to enjoy? Do you have any special interests or hobbies?
I hope to chat to you some more Aimee and I hope that you keep reaching out and sharing. We care so much and are here for you.