Welcome to the Forum, it is a good move as I have the feeling you want to throw away an awful lot of effort and successes.
To get to final year is a pretty large achievement in itself, and while I can see you are discouraged about these fails there is a reason for it - your family's violence incidents. Nobody is going to study well with police turning up and that sort of atmosphere.
So may I suggest firstly you go see student counseling and explain the situation, they may well be able to defer thngs so all is not lost. If by then you have seen a doctor his/her report may help too. Even if you do not use the degree for what it is intended you will have accomplishment, and it can lead to other things.
Secondly the fact your pets died and you were numb is a sign you should at least talk with you GP in a long consultation and say all about what has been happening to you and how you feel. Wanting to give everything up is a superficially attractive process, linked to hopelessness, however in the long term I beleive you will have regrets. Better to get help in coping now.
Nothing you have done is wasted, and has come with a price, here I'm not talking about a HEX debt which will basically look after itself, but in effort and isolation and stress.
Your activities sound wide-ranging, sensible and good for you. I'd not dismiss volunteering just like that . The fact you have been doing it for 2 years is itself a measure of your personality, and an asset. If it is fun -excellent!
With help you will cope better. Things that seem hopeless will become possible again.
Is there anyone in your life you can lean on at the moment? Facing all this alone is hard.
You know you are welcome to talk here anytime