After seven years of part time trading, I came to the realization that for me trading is a 100% mental game now - because I know TA principles well, if I trade poorly, it was due to some mental error I committed.
Dwelling on the past and missed trades is a mental error. If you're going to think about anything, you need to remind yourself of the times you did well, not the opportunities you missed. Human nature is to focus on the negative it seems. I hate myself for not just buying all I could of ES at 800 in 2009. I was new to market then, and didn't have a job (no $). So, your post is not far from that thinking. If I had purchased 5 ES in 2009 well that's 300k. I could be permanently retired and living in the Philippines with that. Just last Friday I bought crude and scalped it for a few cents. It then went up more another 90c without going to where my pre planned stop was. I can find another 1,000,000 reasons I should be angry about missed opportunities.
There are always 2 ways of looking at something - I can hate myself because I fucked it up and should be richer than I am today, or I could say, next time I need to hold on because there was no reason to get out, and I was right. The latter thinking is the only one you should do because in order to achieve something you must first realize it is possible and once you go too far with the former, you will either go crazy, give up, think trading is impossible or can't be done or kill yourself.
...hindsight thoughts are kind of meaningless in trading. 
