This is a most complicated issue ie. a case of ego depletion which can only be satisfied by the approval (parental love) of others. If it's any comfort you are not alone in this area: it is a universal problem and in this era of the "me" generation is probably more prominent than ever before. Obviously it can't be solved by an Email(even one from me), but I will make some points for starters. I believe that your philosophical and psychological insights are accurate and important. You are affected by your employer's (re.parental substitute)opinions for 2 reasons: 1)You mistakenly assume that they are a reflection of reality when they are not; either because (as you say) they just reflect his mood of the moment or his reaction to something you are doing--- not a carefully considered evaluation of the true worth of your efforts. 2) The major mistake is that you attribute reality to the feelings others have about you. It is a good for someone to like me and an evil for someone to dislike me. This is the philosophical error of living in the world of the social reality. You must always remember(and tefilla can help alot in this area ) that the only thing that matters is how you appear in the Sight of G-d; that is the only truth and the only value and the only thing worth striving for in this existence.We should only care about what G-d thinks about us and realize that taking seriously the opinions of others only gives a false sense of security (when positive) or insecurity (when negative).
However simple understanding of these ideas is not enough, for the emotional pull to approval is very powerful. Hence we need to adopt the method of behavioral change advocated by the Rambam. This means exercising very strict self control and not indulging the emotions in this area.If you were an actor I would prohibit you from reading the reviews. You would go a long period of time not indulging the desire to know if people like your work or not. If you heard something positive you would be commanded to exercise control over the desire to luxuriate in the good feeling of being loved. You would be told to stop thinking about it or to recognize the emotional insecurity which is prompting it and realize how fleeting and insignificant praise is and just move on. When you understand that praise is like a drug creating a dependency for more and making you a slave to the whims of others, while harming your soul you will, over time, grow to despise it. At the same time you must react differently to disapproval. You must think over the issue clearly and objectively. If you did something wrong then address yourself to that(ie. be selfish removing a defect is important---eliciting a certain attitude in another person is a waste of time).When the sick feeling of perceived disapproval begins to come over you exercise mental discipline and don't give in to it.Force yourself to gain control over the emotion. Go ahead with your work or get involved in learning as chazal say "if this ugly one grabs hold of you drag it to the Bais Hamedrash".After a long period of exercising this control, together with increased philosophical insight the balance of emotional forces will begin to shift in your favor.