First of all - rip out what you have done. Then re-sandwich the quilt, paying special attention to making the backing as taut as you possibly can before laying on the batting and top. Pin every 3 or 4 inches everywhere (or baste - whichever you like). When you begin sewing, have your stitch length set to medium (probably around 8 stitches per inch), use a walking foot and start in the middle of your quilt and work towards the outer edges. My backs looked similar to yours when I first started quilting and the use of the walking foot and heavy pinning and quilting from the center outwards has worked wonders (have not tried basting spray yet).