2000 matches passed. I spent 5 hours on a train journey yesterday, so I decided to practice offline as best as I could in training mode. As soon as I could, I went online, only to be destroyed in every single match. A Ness player beat me 3 times - I couldn't even land when he kept using PK Thunder, and he would taunt me for it every time I got hit. Then this Captain Falcon came along, and utterly humiliated me.
https://www.youtube.com/watch?v=pVbYU6Aemy4
I haven't learned a thing clearly; I still have no proper Main as no matter who I choose, I get slaughtered by someone who's only just picked up Smash for the first time. Anyone who sees me frequently on here will know that card in my signature never settles on a character. I can feel good about winning one or two games, only to find I lucked out against finding bad opponents, and when I go into a REAL battle, I get destroyed and mocked for false hope.
I have no sense of how to clear my mind, and I always lose. Always. I'm a hopeless case in every sense of the word; the Smash 4 community shuns people like me because I can and will get salty about the fact I never improve. I try harder than anyone to do so. I read all the guides, watch all the videos, practice on the Ladder, and more besides. But I still ALWAYS lose, because in an actual game, I forget it all and forget to even dodge things properly.
2 things I'm noticing, one gameplay related, and one not:
1) You seem to lack an understanding of what "safe" in the context of this game is...and knowledge of many of the fundamentals for that matter. Trust me, this is not to bash you (you posted the vid because you wanted critique right? not just for us to go "oh, yeah, you're right, you suck - good luck with that"), but to suggest that you get back to basics in terms of learning how to maneuver, space (super important for Wii Fit with her wonky hitboxes and whatnot) and defend. It might have been nerves, or it might have been the tilt you were on, but so many of your options in that video were unsafe - your movement, your attacks, your recovery.
I'd advise you to do some research on the topic - broad safe play strategies can be applied to pretty much any character (so you don't have to get hung up on whether you switch mains) and will go a long way towards helping you win. Or at least help you live long enough to give yourself a shot at winning. And if you become overwhelmed by technical jargon, work on one simple thing at a time, e.g., where should I land to not get hit? is this attack fast enough to use when I'm this close to my opponent?
One thing at a time.
As a post-script to this bit - all that time in training mode is good for pure tech skill, but it won't help when it comes to learning how to really play. As much as it blows to lose, you gotta use these human matches as your practice time. And if it's not a tournament, really that's all they are: practice.
2) Everyone gets salty now and then, but Smash is not that serious - the overwhelming majority of us are not doing this for a living and even those who are wouldn't be if they didn't find fun in the game, win or lose. All of the advice in the world will not help you if you don't work on your self-esteem issues (which, if I'm not being presumptuous, seem to extend beyond the game...) and attitude towards playing. Worry less about winning or understanding all the terminology and just try to appreciate that maybe you learned one new thing today - that you didn't fall for something that you always did in the past or that you pulled off a two hit string that you read about online, in-game (and on-purpose).
This is not a game for those who are easily discouraged by others being better than you (the series is over 15 years old and some people have been playing it for that long) or by being taunted when you lose. It's unlikely that your faceless Internet opponent is out to truly humiliate you and certainly no one in this community is out to shun you. It is also not a game for those who don't want to spend hours upon hours (which will amount to way over 2000 for glory matches - which, to be honest, only count for so much) of work - even the "naturally good" players need to hone their skills if they want to be top competitors. If you can't not sweat the small stuff and can't appreciate the tiny victories, then this just isn't the game for you, my friend.