Time, Patience and Practice
High-end retouching requires time, patience and a lot of practice. Keep that in mind, because it's very important. There's a big difference between an image retouched by a professional photographer and an image retouched by a professional retoucher. Photographers often don't have the time to professionally retouch an image; so they don't spend more than half an hour on editing and retouching on a single image, or they hire a professional retoucher to do the retouching for them. Professional retouchers on the other hand won't spend less than an hour on an image, usually 6-8 hours, more or less. Some of the best work is at pixel level, that's one reason why it may take a lot of time. So as we can see, time is a key factor here.
Apply yourself & Be Patient
You have to apply yourself and be patient when learning anything. Remember, professionals were once amateurs.
Learn, learn, and learn some more
You will not be a professional retoucher overnight