Realistically, the moment the game is over, if you're successfully living in the present, then the game and its outcome is in the past, and the outcome of a past event should no longer affect you.
Yes you lost, and yes, you could have played better, but being upset about it changes nothing.
You either decide to work harder and practice (future action steps), or you decide to let it go.
I've certainly succeeded in not being downright angry anymore - I used to hate losing more than anything. But today, after losing a meaningless basketball game, I find myself still irritated, and still carrying a negative outlook with me after the game.
The strongest feeling of course comes from your own assessment of how you (yourself) played.
If you played great and the team still lost, you feel somehow less annoyed (perhaps you're annoyed at the others on the team). But the worst is when you played like crap and the team lost - nothing stays with you like that.
This is of course a powerful affect of the ego - ultimately, you're drawing self value from comparing your own skill to the skill of others. Losing decreases your self value, and winning increases it. The key here is of course to abandon winning and losing as a source of self-value overall (thereby remembering that our true self value is within us, and infinite).
What's interesting is that...I don't want to turn off the positive feeling I get from winning!
It's like a drug - it puts me on top of the world! Again, of course, this is an ego reaction, but I think what I'd ideally like to do is let my losses go, but wear my wins on my sleeve - all the upside, without the downside.
Think its possible?