Try study doing the negative shapes to get the form right. One way to check or correct your own drawing you can just flip the drawing upside down or just flip it with a mirror. You will notice some drawing error like the eyes on the right side is small and etc. and lastly keep on drawing, I know that it will take a lot of time but one day your drawing from six months ago or a year will give you a big impression that you’ve developed a lot of your drawing.