In fact, developing willpower (or rather, discovering it in yourself) is not so difficult. Each of us has it.
If you have to force yourself to get something, you should seriously think about it.
Stop trying to develop willpower, stop fighting with yourself. Your willpower is fine.
Just turn goal achievement into an exciting game.
Never scold or punish yourself.
Your body is not your enemy, and everything it does, it does for your own good.
The laziness and lack of motivation that you blame yourself for is just an indicator that the goal you are trying to achieve is not yours.

Find out what you really want so you won't have problems with your goals.
What is behind your desire to lose weight?
Why do you need this?
For what?
Keep asking yourself “why?” until the body reacts - you will understand when it happens.
So it turns out that you do not want to lose weight (for the body to lose weight = lose, no one wants to lose!), But, for example, to be loved. Or dance (and you are shy when you are overweight), or travel, or wear beautiful clothes.

Set your own goal and you won't need motivation, you will feel inspired already at the stage when you formulate it.

Break the path to the goal into small segments and assign yourself a reward at the end of each segment.
Lost a kilogram - buy a nice trinket, five - new clothes,
ten - arrange a professional photo shoot. Praise yourself for every small achievement so that the path to the goal does not seem endlessly long.

Developing willpower is not the goal. Your goal is completely different.
So go to her. Taking small steps, not stopping, encouraging yourself and rewarding for every small success.
As soon as you understand that the benefits of achieving goals far exceed the immediate pleasures, you will automatically feel both strength and will.
And each time you can set goals higher and higher.
And getting them is getting easier and easier.