This is normal behavior. When they are young and growing quickly they are always hungry. As they age their growth rate slows and as adults they only need to maintain their weight. They're not "picky" so much as they won't eat themselves until they grow obese and die young, unlike other snake species (or even dogs, cats, horses, people...).
I would just keep offering a rat every two weeks, and plan on offering one every three weeks over the winter.
Look at the bright side - your food bill is going down and she'll be easier to take care of since less input = less output.
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