Are you identifying the root of the fear correctly. Not saying you're not, just bringing it up, because I have those vague feelings of apprehension too, but I think at the root is fear of failure, more than fear of losing. Perhaps unanalyzed, they can get confused.
As long as I was fat and not doing anything about it, dieting was always something I "could" do. But what if I try as hard as I can, and don't lose (as a menopausal woman, that has happened to me. I know that is not, and never will be your issue. lol!). Then what? When you discover you are powerless to fix the problem and trapped in a fat body with no hope? That is a fear of mine. Humans do not do well with 'no hope'.