It is impossible to get to my weight without being very bad, very often.

Thank you for a very warm welcome.
As for persuading you not to have something yummy for lunch, there are two options:
First, you could try 'righteous dieter' in which you enjoy choosing the most healthy (boring) option on the menu, only drink water and announce very loudly that you will not have pudding as 'your body is a temple....'. The plus side is smug enjoyment of being Ms Perfectly in Control. The down side is that you may not get another invite.
Alternatively, you could go and enjoy yourself, don't go mad, chat and laugh so much your mouth is too busy to eat. Have something chicken based or a veggie dish and a fruity pudding, dressings on the side.
I think the second option is best, because in a months time, you will remember the laugh you had with your friends, not the pudding you had.
Enjoy.