Remind yourself not to blame yourself for things you could only know with hindsight. Similarly, for past successes, try not to congratulate yourself for good luck: "Wow! I won the bet... this Russian Roulette thing is free money!" Try to take the right lesson from the way you approached the event, not merely from its outcome. The lesson about a new approach is something you can then use in the future.
Other than that, I think the key is: acceptance. It is much easier said than done, but the more completely you simply accept something as unchangeable (as the metaphysically given) the calmer you will be. There's a huge problem with doing this, because -- emotionally -- it is the opposite of ambition (the desire to change the man-made and changeable). Ideally, one ought to be accepting some things as unchangeable, and working hard to change others... but, the distinction between the two types is seldom obvious, and we surely don't want to resign ourselves to many aspects of our lives. Still, we have to start by figuring out -- intellectually - what can be changed and what cannot (at least in our lifetimes, and with reasonable amount of effort). Doing this intellectually won't automatically change one's emotions, but I think its a start.