Hello and thanks for your letter.
Your life sounds very uncertain and tough at the moment. Your mum doesn't seem to be listening to how you are feeling either.
It would appear as though you have thought really hard about what you think would be best for you. Now you've made this decision you're worrying how to make things happen.
From what you have said your dad and his partner are supportive. Would it be worth asking your dad if he could help you talk to your mum again? Would you want that and do you think it would change anything?
Your local social services department may be able to talk to you about what to do next. Possibly your dad would go with you for legal advice, as well as sorting out the financial situation of you living with him.
ChildLine has infomation about relationships and families that might help. Have a look at the advice about home and families. It's important that you are safe and cared for.
Check out the ChildLine message boards on the website and see what advice young people in similar situations have done.
Remember if you would prefer to talk to someone about this you can call ChildLine on 0800 1111 or you can chat in 1-2-1 online with a counsellor.
I hope this helps and I hope things work out for you
Take care
Sam