Wife airing dirty laundry

Dear Housewife,

I have been married for a while, but early in the marriage I noticed that my wife airs our dirty laundry behind my back and to some degree when I am there.
I have told her I know and she denies it and argues. She nags me, blames me for everything, and if i argue with someone she wil not support me and even talks to them behind my back.
But as usual she blames me as I am the cause.
I tried speaking with her but she still blames me, nearly 20 years married, i am ready to walk out. I dont really want to, as she is good and loves my child.