What to do about my Mother wanting to steal my partner?

I have been with my partner for 16 years, she said it would never work and that we would be split up in 6 months. Well now here we are 16 years later with 3 kids. I now find out that my Mum has the hots for my partner and has been trying to undermine me at every turn. Her ideal fantasy is for me to leave and she steps into my place. What the hell do I do?