If you are referring to the Tendler and Carter book, the section 'Improve your C-game' two things are stated. Firstly, "Your C-Game [are] the cracks in [our poker] foundation ... There is always some mental component to our C-game." and secondly "Breaking through mental game problems make it much easier to eliminate tactical mistakes. In order to fix a mental game problem, like fear, tilt, or low confidence, you must first understand whats causing it. The mental hand history is a process that was developed to help players get to the root of their mental game problems."
There is a 5 step process mentioned in the book is:
1. Describe the problem in a much detail as possible.
2. Explain why the line makes logical sense
3. Find the error in this logic.
4. Correct the flawed logic
5. Ask/Answer why the new logic will work.
When you mention a mental hand history database, I am unsure what you mean. The goal of the 5 steps is to improve one's decision making. If one is simply cataloging their mistakes that is not necessarily improvement to mental game, as is intended. Also, when you mention there is no system to work with, those 5 steps are the system.
I've not actually read this book, however the idea seems to me what is referred to by most players as 'leek finding.' Everyone looking to improve does it. So to fully complete my answer I'll share an experience I have had and use the 5 step process as outlined in the book.
There was once this poker player, named me, who was one serious TAG-Fish. Player would raise, usually in position or with a monster holding preflop, and would c-bet about 90% of flops. However, as a TAG-Fish, I would almost always play fit or fold by the turn. If I didn't have a made hand, or 15+ outs, there was no way I could call a lead on the turn. I would check if I could, and be bluffed off the hand on most any river. Leaking money indeed.
Step 1: (not in as much detail as possible for obvious reasons) I am afraid that anyone who called a c-bet was certain to have smashed the flop.
Step 2: One must call tighter than one raises. Therefore their hand is larger than ALL hands that would have folded. If my hand would have been folded to the same c-bet, I should discontinue barreling turns and rivers.
Step 3: One does NOT necessarily fold out weaker hands. Draws with great implied odds don't fold out. Fishy players almost never fold flush draws, straight draws, top pair any kicker.
Step 4: If one calls a c-bet their range is narrowed, but not necessarily connected. What position did this person initially call or raise from. In that seat is my hand a monster, or is it a looser seat (say the button). Compare previous opening hands and calls to current situation. If player is opening wide, he just knows we fold a lot. Doesn't need a big hand to call us.
Step 5: This will work because the opponents who defeat us a a TAG-Fish are opponents that are keeping tabs of previous actions. If we also keep tabs on previous actions at the table we will more accurately represent larger hands when needed, and be harder to bluff off turns and rivers because we no longer assume villain has a strong made hand with a c-bet call.
Perhaps I'm still a tag fish, but this month's results were so much better than last. Now I move on to my next leek, and repeat the 5 steps. The idea is to do this to infinity, or as long as you keep playing poker.