"Why should you....?" "should" is a modal verb, you can't use any auxiliary with it.

It is not entirely clear what you are trying to say. 'Why you should stop being so hard on yourself' (without the question mark) would work as a title of an essay or article. 'Why should you...' makes sense, but probably means the opposite to what you intend as it implies that the person should continue being hard on himself. 'You should stop being so hard on yourself' is probably what you mean. As said above, no auxiliary verb

I would say "Why are you being so critical of yourself? You should stop being so hard on yourself."