I used to be the same way - not to be
that guy, but what helped me through it was a daily routine of ten minute meditation sessions. Now, whenever, and if ever, I feel the anxiety come on, I just take ten slow, deep breaths; focusing on the sensation of the breath as it passes through my nostrils, flows down my throat and expands my lungs. It helps to clear my mind, allowing me to focus all of my attention on the game.
You don't
have to meditate, but I think everyone should - mind you, stopping to for a few deep inhalations will work wonders, or at least help to some extent. Really, the best advice I could give you right now,
wertyou2, is to learn how to live in the moment of life. Too much of your mental activity is (I assume) consumed by a growing resentment/self-hatred to yourself and losing. Basically, the reason you have an issue in the first place is because of the way you interpret life; you built yourself high up upon the ladder and then you enter a tournament, and you lose - all of reality comes crashing down as the appalling truth is learned that you actually aren't that good.
It's right after this realization where you come to a cross roads. On your left is: succumbing to your emotions, thus conjuring negative thought, thus no progress will be made during its duration. On your right: you mourn the loss, for it was so very tragic, but after that is done you ask, "Why? Why did I lose; what was I doing that wasn't working?" You need to step
way back and look at not only how you play the game, but also yourself; why do you choose to play the way you do? ect. Self-reflection is so important to growing as a person, and subsequently growing as a smasher.
To put it simply, don't give up. Giving into your emotions only leads to destruction of the self; no good will ever come of it - it's fruitless, just stahp. View your losses as an opportunity to better yourself; as an opportunity to prove your worth, and so on. Never can anyone sit down and say, "I'm done getting better at Smash Bros, I have nothing left to learn." whether or not you realise, there is always room to improve and learn. Even someone like, Mang0, for example - you could say he has nothing left to learn, but he does. Humans are adaptive creatures; we're always changing throughout our lives. The way Mang0 plays the game today might be entirely different to how he plays it tomorrow - one day he might discover a unique little thing that alters how he plays the game in a small way. I hope you see the point I'm trying to make here~~
Deep breaths, keep your back up straight and live in the moment.