I think you are doing great at day 9 on your first cleanse. I'll be starting my 8th next Tuesday. I think your back pain can be associated with the healing phase, since you are so far along. I am not sure that I would stop the fast on day 10 if it is still hurting. The body might be trying heal any damaged tissue, etc., so it might be good to work though it and try to fast until the pain diminishes.
However, if you do stop tomorrow at day 10, you can certainly do another in 3 months. I do a 10 day Master-Cleanse every quarter. I did 3 in 2005 and 4 in 2006.