My biggest problem has always been losing to people that I have a legitimate chance to beat.
It sounds kind of obvious, but I've always looked at it like this: there are three phases to being good at Smash (or any competition):
1) Beating the people you should (Ex- If you play random noob #3, you should win that match)
2) Beating people you can (Ex- Someone around your same skill level)
3) Beating people you can't (Ex- The players that are better than you, or at least eveyone else thinks they're better than you)
I've been stuck on phase 2 pretty much my whole career. I've 4-stocked people in the first round of a tournament, only to lose to someone who used to be really good at Melee but hadn't played in months in the second round. I just choke. Or I make incredibly stupid decisions that cost me sets.
I'll never forget a tournament where I played a Luigi. He beat me the first round. The second round we went to FoD, and I three stocked him. The final round he went random for stage, and we ended up back on FoD. And he beat me by one stock. It's things like that which have kept me from really achieving my potential. I'm just a choker.
An example of a bad decision was the last time I played KevinM (<3 KevinM!) He picked Sheik, and for some inexplicable reason I picked Peach. I'M AN IC MAIN! WHAT WAS I THINKING?! So he beat me the first round, I came to my frakking senses and picked ICs the second round and counterpicked with FD, and in the last round he picked Marth and Yoshi's Story, and I lost by one stock. If I'd won that first round then I would have had the last counterpick. Just poor decision making, and I paid for it.