Why do we remember criticism more than praise?

This is what psychologists call the 'negativity bias', they think we've evolved this to keep us away from harm. Basically, we're more likely to remember scary of painful events more than happy ones - remember unkind events more than kind ones, process bad news more thoroughly than good. As for self-perception we're highly motivated to avoid a bad image so react with stronger emotions to criticism more than to praise. On the other hand we can overcome this negativity bias by over-estimating their own positive qualities.