How do i deal with false allegations made against me?

I need help, i am being accused of child abuse against my fiance's kids yet they are not true, i have people who can prove it's all lies but social services refuse to believe me and i don't know how i can stop her from lying, she was supposed to be my best friend, i put a roof over her head and now she's stabbed me in the back and i don't know why, if anybody can help me then please, i beg of you to help, i can't eat, i can't sleep and worst ofall i fear i may lose my fiance although he is standing by me because even he knows it's all lies, i feel trapped and alone with nowhere to go and no-one to turn to, what can i do to prove she is a liar, she has really hurt me and all i really want to do now is curl up and die, i was abused as a child so i know how it feels so why would i want to put a child through that same pain. I just don't know what to do