Why do I keep self sabotaging?!

[pennylane2015]

Been doing slimming world since April. Have lost 11lb, have a stone to go to target. My baby boy is 6 months old.
I know slimming world works. But it seems, I have a great week, stick to plan, loose and then my motivation wanes for a week or two then I get back on track. I know the only person who can change is me, but I'm struggling so much. Been away for a friend's 40th and completely over ate. I felt I looked chubby and frumpy on the pictures.
I know what I need to do, but seem to keep ruining it on myself

www.mumsnet.com/Talk/slimming_world/2439867-Sabotaging-self-Help

Here's a same question, maybe the question will get a few more suggestions.

I'm rubbish when the weekend comes.

Thanks, I guess not much people can say really. This is just something I must do