They say a journey of 10,000 steps starts with a single step. And then you have a journey of 9,999 steps, which starts with a single step. That is, there's never a time when you take 10,000 steps in one go, or when you do an entire journey in one go, the whole journey ends with a single step as well.
Given that the entire journey is single steps, if you take one step and then beat yourself up because one step is nothing, and tell yourself how much of a loser you are for only moving one step, and how other would people disapprove .. that makes the entire journey thousands of steps where you beat yourself up because each step isn't enough. How are you going to face, and endure, such a miserable journey?
By contrast, if each step is a rewarding piece of progress, which you cheer yourself on for, then (in a recursive sense), the entire journey ahead of you unfolds into thousands of rewarding, cheerful pieces of progress encouraging you forwards.
I myself didn't lose half my bodyweight because I was happy, quite the contrary, I hated --and still hate-- my body.
You've worked hard to go from self-hate to self-hate, through thousands of steps each full of self-hate. What kind of progress is that, really? What kind of life is that? In a fraction of the time and effort you could have gone from self-hate to self-approval, without changing anything except thoughts and without doing anything except thinking.
I'm equally sure that happiness will not make one successful.
If you were happy being fat, what use would you have for "success"? What if you die half way through the journey and never reach the end - is success a goal that only exists at one point in time, and if you don't live long enough, you can't have it? Why not feel happy and successful at every moment of calorie counting, grateful at having the opportunity to buy chicken and salad instead of fried chicken and gravy, happy at the scenery around you when you go out to exercise, instead of filling each moment with self-hate and bitterness and what have you?
If every moment you play a tune, you hate how badly you play, and it drives you to play more precisely, but you hate playing and feel bad afterwards, are you better off or worse off than someone who plays imprecisely, but loves every moment they play and can't wait to play more and feels better afterwards?