"Should" statements evoke a sense of guilt and failure, "I should be a better daughter," "I should be more successful," "I should be smarter," I should lose twenty pounds." By undermining who and what you are, these statements can begin to fuel emotional struggle.
While it may be true that you can improve yourself in some way, when you tell yourself you "should" improve, you're telling yourself a negative: you're not good enough right now, and you can only be good enough if you do this or that.
The healthy alternative is to avoid should statements by replacing them with more positive assertions such as "I want to be more successful," "I want to be more attentive to my mother," "Maybe I'll join the gym and quit eating all that fast food." These alternatives don't negate who you are now. They support growth and improvement based on who and what you are.
When it comes to living a more positive, happy life, remember: encourage don't discourage.