Lyr - write it down. Write it all down - your analysis of why you lost, and how you can prevent yourself from making the same mistakes in the future. Writing is cathartic. Once you have all your thoughts on paper, and have revised it a couple of times, then you can walk away from it more easily.
But be sure to put a good couple of hours of work into it, so you're satisfied that you've covered every angle.
This helped me improve my 40k game when I was first starting, and getting regularly shellacked at tournaments.