That's like asking a chain smoker to just stop buying cigarettes and have an apple instead.
We have to get to terms with the fact that obesity is an epidemic that has to be treated the same way tobacco has.
Stop putting so damn much sugar and salt into processed food. More physical education in schools and pre-schools/kindergarten (it's appalling how much the motor skills of children have deteriorated compared to previous generations). School gardens for every school so children can learn from an early age what food even is, how its grown and what it actually tastes like.
It's a bunch of measures that need to be taken and while it might be simple for some people (like you for example) to change their lifestyle, it's really hard for most.
> Just cut out the sugar
This is a great example for why it's so hard: most processed food (and even bread) contains tons of added sugar without the consumer even realising it. It's not necessarily the donuts, the chocolate, or candy bars. It's the sodas, fast food, bread, parfait, fruit drinks, the canteen food, etc. The practise varies from country to country, but some (e.g. Mexico) had to pull the emergency brake already and limit the amount of added sugar by law.
Since sugar is highly addictive, it's hard to let go of these products, especially since most of us are conditioned to primarily consume highly processed junk basically from birth.