I couldn't find the spell here, and before I go on I would like to say something.
No spell will make you lose weight suddenly/without work done to back it up. You are not going to lose weight without changing your diet and exercise habits. A spell will simply help increase motivation and give you a push in the right direction.
You seem to know the basics of the spell of the spell you are looking for, so I encourage you to create your own spell with the same basic idea if you like it. You will likely get a more personalized effect, and finding the spell again may be next to impossible.