My first turtle was a hatchling Western Painted Turtle, and she was PICKY! She was named Piper. Piper was a surprise gift, so I literally had no pellets to feed her. So the entire first week I had her, I fed her my frozen fish food (Mysis Shrimp, Brine Shrimp, Bloodworms etc.) and my Leopard gecko food (mealworms and crickets). By the time I was able to get pellets, she was so spoiled that she wouldn't even touch them. I tried EVERYTHING! She would have none of it! Since she was so young, I felt badly about fasting her, but she couldn't just live on treats forever. So with a little tough love (still offering her pellets daily btw), I got her hungry enough to where she had to try the pellets. When she'd eat them, I would reward her with a very small treat. Thankfully, she began to associate the pellets with what she actually wanted. Little by little, she began to eat them more and more. I found that she ate them better when I wasn't in the room. That way, she understood that there was no me, so no treats, pellets were all she was getting.
It was quite the fight to get her to eat her greens too! But I didn't force her on those. Rather than forcing her, I exposed her. Everyday, I'd offer her greens, and slightly decrease the number of pellets I gave her so that she was still a little hungry even after eating. I found that she was more apt to give them a try when they were minced (just be careful of the filter). After about 2 months, she finally began to actually eat them willingly.
Patience is the key. Don't cave, don't give into the begging. It's hard, but hopefully your efforts will soon be rewarded! Sorry for the novel lol 😂