You are NOT a failure. One slip up does not constitute as a failure. Everyone will slip every now and then. What you need to do is evaluate what triggered you to do it. Think about it and try to come up with a way of coping with that trigger. I know it is hard but don't be so down on yourself. Slip ups happen to everyone whether it be SI or trying to quite smoking/drinking. It's ok, it really is. Getting depressed and being down on yourself may cause you to do it again. Try to think in a positive manor and look at the incident objectively and see if you can find a solution.
I wish you luck and please let us know how you are doing.
Also,
YOU ARE NOT A FAILURE!!!!!