See that's where I get confused coz I eat pure sugar and carbs and fatty foods but can not put weight on :-/

I crave sugar ALL THE TIME too!! I really wish I didn't!! My weight gain is getting worse and worse though.

I have tryed eating a high sugar high calorie diet but I tend to put a few pound on then my weight stays the same and then dropps again

Yep to both.
Trying a new diet now.