I've always been trying to have a less for dinner these days.

"I've always been" and "these days" have contradictory meanings in this sentence. "I've always been" implies sometime that's always been true, while "these days" implies a recent development.

Because I've put on weight during the winter, so I have to lay off some sweets and foods.

I've always been trying to have a lessalways have a smaller dinner these days.

Or you could say have less food at dinner, smaller portion sizes

Because I've put on weight in the winter, so I have to lay off some sweets and foods.

What Do You Do Tto Stay Healthy?

Prepositions aren't usually capitalized in titles. If you're ever unsure, you could use a website like this to check: https://titlecaseconverter.com/

I've always been trying to have aeat less dinner these days.

Because I've put on weight inover the winter, so I have to lay off some sweets and other fattening foods.

I've always been trying to have a lessight dinner these days.

Because I've put on weight over/during the winter, so I have to lay off some sweets and fatty foods.

When you start a sentence with "Because", you can't add "so" in the second part.
If you had removed the "Because" at the beginning, you could have said, "I've put on weight overthe winter, so now I have to lay off sweets and fatty foods.
I added the "fatty" but any other adjective would be find. Food on its own is unclear.