I am stringing my own rackets. I string a ton of rackets, and I am the only one to whom this happens. They break in the center horizontally wise, just above the middle of the racket.You are breaking a poly string once a hitting session? Somethings not right. Who is stringing your racquets ? where is it breaking?
-the 16/19 pattern of your current stick is ,"to open" thusly you are breaking strings so quickly
-you are stressing the strings too much with your current setup, level of play and swing
-changing strings wont do much good
-try to find a 16/20 or 18/20 frame you like
-racquet change should reduce the string breaking to a more manageable time frame
-edit: or a tighter pattern 16/19 racquet (example head.rad.pro)