My "casual days" were back in Brawl, and at that time I actually had a good idea of how to play and what good strategies were. The problem is, I was so unaccustomed to grabbing that I almost never used it, and the second someone else used theirs, I would remember I had one and then spam it for the next minute. I was also one of the many who played really aggressively on their controllers and never knew that tilt attacks existed, so whenever I tried to do a smash attack and accidentally did a tilt, I was always disappointed or confused.
My controller scheme was also cringe-worthy. I started with the terrible sideways Wii Remote, and when I taught myself the Wii Remote + Nunchuck, I thought I had mastered the game. If only I had used my Classic Controller I bought for Mario Kart Wii back in Brawl...
I also played against the CPU on gimmicky stages way too much, so I was terrible against real people. I've improved on that now.