I no longer have my 360 around, and even when I do I rarely play it now (since my arcade stick only works with my PS3, I'm waiting for BBCS) online. But I do have a friend that I can always practice with IRL (who's moving away ). He's better than me in so many ways and I always get my ass handed to me about 9 times out of 10, sometimes more. But there are two things that I always keep in mind. First is that it's a game, a competitive one, but I'm playing to have fun not simply to win, and second when I get too worked up about losing, it's time for a break. Just give myself some time to calm down and come back in like 30 min or whatever. When I come back with a clear mind, it's much easier to learn about my mistakes because my frustration often clouds my ability to learn from my mistakes well.
I've edited this post so many times. I should have payed more attention in English.