How do I handle developmental delays and tough behavior in kids?

You have come far. You can support a child's growth day to day, and that is real skill. What stumps you now is the harder edge: the delays that do not follow the book and the behavior that does not settle when it should. It makes sense that this is where you feel stuck. It is genuinely the hard part. A delay or a hard behavior is almost always the child telling you something they cannot yet say in words. It is not a wall to push through; it is a signal to read. You will not always decode it right, and you are not meant to alone. This is the point where good people ask for other eyes. Pick one child who worries you and write down what you actually see, plainly, for a week. Then bring it to someone who knows more, a specialist or a lead. You are not admitting failure. You are doing the exact thing a skilled person does when the problem grows past one pair of hands.