I heard before that people are not usually successful the first time they try to lose weight or quit smoking.
But, with each failure we should learn something about what is keeping us from our goals, and try to change it. Then next time we try maybe we won't make the same mistakes.
If you don't already, try keeping a daily journal. I keep mine in my notebook on the computer because it's easy to update and edit. I put my weight, inspirational phrases, my goals (not just in weight but also things I'm looking forward to, like going on a cruise and wearing a bikini again)
I also plan rewards for myself; when I reached 20lbs lost I got an italian charm bracelet.
Most importantly I put what I'm frustrated with, the challenges I have and my plan to overcome them next time I'm presented with them. Like, if my family orders pizza I will go take a hot bath with all the goodies; candles, music, facial, bath salts, shave, lotion etc.
Usually by the time I'm done the pizza is gone and I don't care to have it anymore anyway.
And, if you do slip and have something off your plan try not to give in. Put on the brakes! Go brush your teeth. Do something active if you can.
And write in the journal what happened and why you think it happened.
Most importantly, don't give up! If you have a bad day, do like Nancy says, don't do it two days in a row.
This is a long post, but those are the things that really help me when I'm feeling really weak. If you take one day at a time you will succeed! Just as every journey begins with a single step, every pound lost begins with a single shake. We are all in this together, You have a whole group of people who understand how hard it is, don't be silent, let someone know what's going on so we can help.