Rizzen, you are only human. Everyone feels they have failed at something when really they havent. its just how we are, to be critical of ourselves. what makes us mentally strong is whether we can see past the bad stuff and look at the positives and grow from them.
mental well being is just as important as ones physical well being. i believe that bodybuilding is just as much mental and it is physical. it takes strength to push that little harder, go that little further and keep a smile on your face and be happy about it. lifes hard, but what would life be if it was a breeze. the tough times are what makes life interesting.
something small i do in my diary/journal is something i call 3BT (3 beautiful things). everyday you write down 3 beautiful things of that day. it can range from the nice weather to someone doing something for you. it puts a positive perspective on everything, even when bad stuff turns you down.