We regularly have tea/supper at my mother-in-laws on Sundays. She loves to bake for others. Over time I've learned to take fruit and oatcakes, but I still end up eating cakes or buns or scones every time. And then I feel rubbish later or on Monday mornings. I'm feeling very fed up about it like I'm not good at taking care of myself. :-( thanks for reading.

You are just starting the program: h6) It would be all right to be kind to yourself. This is why you are here. Scones with MIL are not part of step 1.
You can just work on breakfast. Sunday will come again and if you like you can make a plan. now. But honestly, this is a step 7 issue.

Kathleen