Though I don't really know for sure, this may be a mentality problem. I went through something sort of similar when I got into the game a few years ago. Of course, things panned out differently for me, but I also had problems with being outplayed.
I'd dabbled around in other characters before, but my first real main was Fox because he's good and I like pushing buttons fast. I grinded out tech skill for a few months over the summer (I would play with my friends during school) without playing against anyone. When we all started playing again, I destroyed everyone because we were all terrible enough where tech skill basically hard countered us.
Over the next few months of all of us playing each other for a few hours a week, everyone started to improve. I still had the best tech skill by far, but I was starting to do a little worse. I've always been and I still am definitely better than the rest of my friends, so I almost never lost and still rarely lose games. However, I stopped destroying them by so much, which frustrated me. I'd developed the mentality that I was better than the rest of them because I could push buttons much faster. I was getting outsmarted, and that's why I was struggling more.
I ended up switching characters a ton; I've given real effort to half the cast at least once, and I always had some success because I was better than my friends. When I switched characters, I would destroy my friends until they got accustomed to the matchups. I ended up switching characters every week or two weeks just so I could keep winning by a ton. I could push buttons faster, so I was better. I was so much better that it would be terrible gameplay to not completely destroy my friends.
I started playing a lot of netplay. I mainly focused on Peach and Samus because they are my two favorite characters to play. Netplay forced me to slow down since I couldn't do my tech skill very well with buffer and occasional stutters. I started winning games after I learned how to outplay the opponent. I remember the most surprising thing to me being how good dash dancing is at baiting out aerials. Instead of just going guns blazing into the opponent with horrible approaches and crazy tech skill, I learned to actually play neutral. It wasn't until I slowed down and started thinking that I realized that I struggled against my friends because they would outsmart me.
You might feel like you're stuck in hitstun and like your friend escapes all of your stuff because he's outplaying you. He finds himself good positions that make him feel harder to hit and that make you feel like he's always hitting you with stuff. Record some games and review them. Find out exactly why and how he's finding himself in these advantageous positions.
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