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It sounds to me like she has impulse control problems, which can become a medical issue along the lines of ADHD and should be discussed with your pediatrician. You'll want to know what to look for so that you can keep an eye out for that if it develops.
You're doing good things already, but you need to know that good techniques only work for a very short time with kids her age. The trick is to keep changing them up. Reward charts that change color and shape every 2 weeks are great. Every day without an accident gives her a marble in a jar, nickels in a cup...things like that. Changing the shape, words, colors and prizes will keep her attention.
She's very young, and this will take a long time for her to learn and hold on to. It's like manners: we tell our children a thousand times to say Thank You over the years before they pick it up naturally. This may be like that with her. Using a memory marker like, "Look before you Leap", or pointing to your head and saying "Think, think, think" can help also.
She'll learn, but it will take time. Just remember that the best interventions only last 3-4 weeks until they're about 10. Hang in there - she's young and impulsive, and most children grow out of that.
My best to you.
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