Ok. Firstly, huge hug for you. 
You've lost your mum in a very upsetting way, and you've been in a horrible situation which has escalated out of control and how your relationship has crumbled.
And of course your mental health is suffering and basically... you are not ok right now.
So, I think you need to focus on yourself right now. Give yourself some time and space away from the stress and focus on getting yourself a bit better before you make any decisions.
Can you get an 'emergency' appointment with your GP so you get seen tomorrow? Or do you have any specialist or community mental health input you could access?
Please treat yourself kindly and gently. And please, don't make ANY decisions right now. Give yourself some space and time and tell your dp and anyone else who needs to know that you aren't making any decisions, and no decision doesn't mean any kind of default decision has been made.
Please try and put your step daughters behavior out of your head for he moment. Don't deal with it as you're not in the right place to cope with it right now.
It's like the airplane safety demonstration always says 'put your own oxygen mask on first before helping anyone else'... your oxygen mask isn't on right now sweetheart, and so how can you possibly deal with a screaming irrational teen with problems of her own?! I know it sounds the wrong way round, but tend to yourself first, and you'll find the other problems have shrunk a little.
I'm worried about you. I know we're words on a screen etc, but you're going through some really traumatic things right now and with your bipolar being triggered and unbalanced, its making it super hard for you too. Life can be crappy like that, making everything that much harder just when you really need everything to stay together so you can cope with the other crap happening.
You are grieving. It's a BIG deal. I've lost my dad and sister and it's so very hard to live through. Losing someone so close is going to shake your world. And you can't 'get over it' as quickly as society or others think is reasonable... my advice is to recognize how effected you are by your loss and to remind yourself that everything is going to be felt through the lens of your grief.
Sorry I'm not great at small posts! Here it is in a nutshell:
Be kind to yourself
Give yourself space to grieve
Focus on your mental health
Tend to yourself first
Don't make any decisions
And take all the time you need


