Training by yourself is great for learning tech skill but if you want to be good you have to play human opponents. Humans do things like switch up DI, read approaches, know character gimmicks, and sometimes throw out unexpected options. Everyone knows how to read a CPU but when you get to an intermediate level its as much about reacting to, adapting to, and then predicting / reading your opponent as actually playing the physical game itself.
In addition you really need to have your fundamentals down flawlessly. 2015 is a far cry from earlier days of smash when you could get away with a missed L-cancel here and there. The advent of things like the 20xx training simulator means that people know the exact spacing for every character's nair out of shield, they have their chain-grabs down flawlessly, they know the hitbox / framedata for everything (if they aren't lazy), etc.
When you don't have your fundamentals down completely to get into situations where you can see what's happening on screen, know how you want to react and how to move optimally, but you simply can't get the inputs right or you flub a command at the crucial moment and you end up not being able to make your character move in the same way that you can see it in your mind's eye. All of this isn't even taking into account tournament jitters and nerves.
Practice up your fundamentals, learn your hitboxes and framedata, practice your character's offensive and defensive options, and learn / relentlessly practice your character's gimmicks whatever they are. Just because people know about your gimmick doesn't mean they can't still be caught by it and the momentum can shift if you land it and get into your opponent's head.
Happy practicing!
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