How to tell my mom t o stop interfering with my son?

So, I have a 3 year old son (whom I love). I am a single mother, 26 yrs old. The long and short is: my mother who lives locally was an alcoholic for the younger yrs in my life and therfore missed out on most of mine and my brother's(18) life. So as a result she beleives that she has a 2nd chance or something with my son. She spoils him rotten, undermines my authority while we are all together,,, Basically has told me in not so many words that she doesn't care to see me, only my son. She is so childish that if she askes to see him(only, never US) and we already have plans, she gets so upset and throws a tantrum and treats me like crap!(guilt trip) She is so dramatic that having an adult conversation with her is virtually imposssible. What can I do to help her understand that she is being overbearing and that this isn't her 2nd chance in life to make it right with herself???