You are doing such a great job! 21 days is quite an accomplishment and carry around a banana is a great idea.
I hope that you make it through but don't be disappointed if you don't, you've still accomplished a lot.
Last year I did an 18 day Master-Cleanse that was planned for 21 days. I just couldn't make it those last few days and I caved and had a smoothie and then juices for the remaining 3 days. I was really down on myself for a while until I realized how much good I had done my body and what an accomplishment 18 days really was!