Absolutely livid...

My future mother in law is an actual nightmare, I've been with my partner for 3 years and for at least 2 of it we haven't spoken, I'll start from the beginning when I first met my partner I was on the pill (aged 19 was very forgetful and reckless didn't think of worse case scenarios) basically I had forgot it for a while and fell pregnant within about 3 weeks and unfortunately had a miscarriage at around 10 weeks I had my scan letter and a doctors appointment which my boyfriend attended and she still tried to say I was lying, since then she's hated me and found every excuse under the sun to make it well known, but since having my son in November 2014 she has seen my son maybe 15 times never once seen him alone and never will as she is well known for drinking excessively, smoking pot in her house, and generally being extremely abusive at random moments so after while she make yet another excuse because I speak to my partners step mother she would stop speaking to me again.., and didn't see my son from January 2015-October 2015 then attended his christening in the November caused a massive argument with me and left and didn't see her again until Feb 2016, now she has recently seen him as I made the effort to take him to her but the past couple of days she has gone completely mental posting photos of me on her Facebook with an abusive status above getting people to share it and has gone to the extreme of commenting on a photo of my friend who passed away in November when my son was born stating "how did you enjoy her funeral?" In an evil way along with commenting on a photo of myself saying I don't know how you smile after what you've done to me I won't deny responding with I feel no guilt after how you've treated my son,  I don't know what else I can possibly do to keep her away from me? Or my son? The women's threatening to beat me up in the street.. And now I question whether being with my partner is truly worth the grief I get for something so knows nothing about? Is there anything I can do? Or does anyone have any advice? Please help me I'm at my wits end