Don't feel bed, if SS isn't for you, try SS+! some people will have an SS+ day when they feel they need it, and if you needed it to keep on going then do it!
If it's the food abstinence that you're going for then try forgive yourself, and move on as tomorrow is another day. I've also read at a VLCD is a VLCD, 440cal or 790cal, you're expected to loose around the same amount of weight. Also, remember this is a marathon not a sprint, one bad day/one slip does not mean you're never going to get there, we'll all get there in the end! xxx