DEAR READERS: After 21 years writing the “Ask Amy” column, I’m announcing that I’m leaving this space. My final column will run at the end of June. I’m healthy, happy, and 64 years old. This is a decision I’ve been wrestling with for more than a year. When I was first hired by the Chicago Tribune to write an advice column after Ann Landers’ death, I was a middle-age, single mother. My daughter, Emily, and I moved from our long-time home in Washington, D.C., and relocated to Chicago. My welcome to Chicago was to deliver a solo performance of “Take Me Out to the Ball Game” in front of 35,000 bas…