I. Need. Help. Please.

I think 你 should sit your mother down and explain how you're feeling. Explain to her, that 你 are 15 now and quite capable of travelling on your own, 你 are responsible enough to know about the dangers of the world, and she can trust 你 to call her if 你 need her. Also explain, that just because she no longer loves your father, she has no right to say horrible things about him to you. Whether they are true 或者 not. he is your father, end of, and if she can't accept that and let 由 gones be 由 gones and be civilised about it, then i'm sorry but she is very immature.
Explain that because your mother keeps saying these things, 你 feel intimidated about wanting to defend him in case she's angry and that neither parent should try to force 你 to choose.
Just tell her that 你 understand why she could feel this way towards your father, but it would be nice is she restrained those feelings away from you. 你 don't want to hear these horrible things about your father, as he's your dad. If she comes up with the excuse 'but 你 don't know what he's like' then just explain that 你 feel your old enough to find out for yourself without any preconcieved ideas out of spite.
Just tell her that with the way she's 表演 你 feel very disappointed and horrible and that she's trying to stop 你 from seeing your father.

If talking to her doesn't help, try talking to your mothers new partner, see if he can make her see sense. If not, what about your dad 或者 your dad's new partner?
你 could also try looking in your local phone and address book to see about family legal aid and find out your rights about your parents divorce and whether 你 have a leg to stand on if 你 wish to go to court against your parents to actually sort this out.
How about councelling, not for you, but for your mother? It may help her. Family councelling could also help 你 get your feelings and ideas across to your mother without her rejecting them without a proper explanation.

It sounds like your mother has alot of hurt feelings that she has not let go of. It is sad that she puts it all on you. I agree that 你 might want to try to get another adult to mediate between 你 and your mother. Maybe that way she would see how unfair it is to talk badly about your dad around you. I don't mean to pry, but 你 did not say what country 你 and your mom live. In parts of the US, a child may decide who they wish to live with at age 16. It would be unfair to put 你 in that position. For now, try to keep in contact with your Dad best 你 can. Try to remember that your mom is angry with your dad. 你 should base how 你 feel about him 由 your interactions with him. It's hard I know, but try to treat your parents with respect and maybe your mother will listen to you. If 你 have a school consoulor 或者 minister 你 trust, speak to them. I hope things work out. Let me know if I can help.