New mom- trying to start feeding schedule

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Needing advice. So I am trying to start my newborn on a feeding regiman (every 3 hours) and I feel AWFUL waking her up!! But if she sleeps for more than 3 hours she wakes up so angry she will cry for HOURS. So I try to be super sweet and gentle when waking her because she is so peaceful. But it's hard to wake her. How do you wake your newborn for a feeding?