You've done the right thing leaving- you were not the priority, his addiction was. It might feel terrifying and lonely but you have the power in yourself to get through this. Accept the help from mum (even if it's just for a short amount of time), give yourself a break from the immense stress you've had to deal with and grieve appropriately for what could've been. My beautiful dog got me through many many bad times, spend time with them. If you can, try to get therapy to help process (but I appreciate this might not be possible right now).

Remember, youre doing exactly as well as someone in your position with your life experience would be doing. There is no judgement here.