A search at AMG finds 50 recordings of at least 10 different songs called "Keep It to Yourself." Memphis Minnie, Lonnie Johnson, Bessie Smith, Sonny Boy Williamson, and several others all recorded songs called "Keep It to Yourself."Amy Rigby isn't listed there, but CDNOW says that she wrote the song herself, along with Bill DeMain, so apparently her song is different from all the others. Alas, CDNOW doesn't have a sound sample.
Amy Rigby's own web site gives lyrics for several of her songs, but not this one. Maybe if you'd email her a request, they'd post the lyrics or email them to you.
I found a quote at http://www.tennessean.com/entertainment/music/archives/02/05/17781492.shtml
The anthology's only new song is Keep It To Yourself, a co-write with Bill DeMain in which a lovers' conversation about an old boyfriend ("You say you'd like to kill the man who broke my heart") takes an unexpected turn ("Me, I'm trying so hard to forgive/ But here's his address, here's his picture/ Here's the make and model of his car").