You're making yourself fail before you even start. Saying you WILL mess up when you're cutting the material is not an effective way to be productive. Maybe if you take a more laid-back approach, saying to yourself that "if" you mess up, so what ... there's plenty more fabric to do it right and you can use the messed up material for scrap quilts. Go easy on yourself. You are frozen in self-doubt. None of us are perfect!