One a hot summer day, after finishing a solid morning of work, I stopped by the supermarket on my way home from the library, with my backpack full of books.

I wanted to treat myself—maybe a dessert, something like a Häagen-Dazs ice cream.

I wandered through the shelves and freezer cases, but thoughts kept popping up such as: “I already have yogurt at home,” or “That’s way too many calories.” In theSo I end, Ied pickeding up a small pack of juicy-looking plums—firm, fresh, and in season.

Your original phrase makes sense and can be used. This is just my minor suggestion to create a smoother transition!

It turns out that what I really wanted as a treat is something natural, seasonal, and nourishing.

Something that won’t make me feel guilty afterwards.

I’ve worked hard to lose weight and lower my body fat, and I want to keep it that way.

You can also say, "reduce" instead if lower and also, "I want to maintain it that way" also works! It has the same nuance.

In the end, the best reward might just be the feeling that I’m proud of the choices I’ve made—and of who I am right now.

I wanted to treat myself—maybe a dessert, something like a(a pint of/cup of/scoop of) Häagen-Dazs ice cream.

For this sentence, you can word the latter half either like:

"something like a cup of/pint of/scoop of Häagen-Dazs ice cream"

or

"something like Häagen-Dazs ice cream"

If you use "a" before "Häagen-Dazs", you must add the amount or measurement of the ice cream. It may be simpler to just say "something like Häagen-Dazs ice cream" and remove the "a" altogether.

I’ve worked hard to lose weight and lower my body fat, and I want to keep it that way.

What you've written is absolutely fine, but I think it's a smidge more natural to just say "lose weight" or "lose weight and lean down".

People tend to speak about lowering their body fat percentage in the more technical sense of bulking and cutting, and then I think it's almost always by saying "body fat percentage" and not just "body fat" as a quantity.

I wandered through the shelves and freezer cases, but thoughts kept popping up:, like “I already have yogurt at home,” or “That’s way too many calories.” In the end, I picked up a small pack of juicy-looking plums—firm, fresh, and in season.

Whenever you use a colon [:], you usually list whatever comes after it using a semicolon [;]. I personally use these things if I have three or more things to list with a given topic, so maybe you could try testing that in a future entry.

Something that won’t make me feel guilty afterwards.

I've found "afterwards" in books and things like that more often than "afterward," but honestly, either can work.

Ever since I read "Atomic Habits" by James Clear, I find it hard to buy snacks on a whim.

If you can, italicize book titles. Otherwise, add quotation marks around them to note who created what.