The simplest answer I can give you is to play against more people and make your own notes as to what works and what doesn't, which should come to you subconsciously with experience. From your description alone you're too predictable and are not great at reading the opponent. I had a similar feeling at my first local tournament. I main Jigglypuff and every Fox I played was too fast and overwhelming with aggression which I fall apart against. There was no time for me to collect my thoughts and everything they did was unstoppable. Sure, I could just "learn the matchup," but that takes a very long time playing against good Fox mains just so I can become proficient at one matchup, when there are also many other matchups I have to be accustomed to at the same time. It is a learning experience you get use to.
Watching YouTube videos and Twitch vods about what combos work at certain percents or what to do under certain situations seems pointless. I use to think that if I watch Hungrybox videos I would become a Jigglypuff master and be able to out-space everything and edge-guard a Fox Top players know what is safe and what is not and avoid them. Keep in mind, there aren't many players who are at the top level of play in most local tournaments. Most of the players don't have the complete grasp of all of their follow up options, how to read and when the optimal time to punish is. All and all, everyone makes mistakes and you have to wait for them, you have to learn what a person does wrong and beat them up for it. You have to also keep in mind of your own mistakes and make a strong effort to avoid them and use a different option (A.K.A. a mixup in simpler terms). "You don't have time to think. Because if you think, you're dead." Though you have to keep in mind of your actions, it is really tough to make plans in the midst of the high speed combat.
One final thing, don't play a match and have a losing mind-set or have the fear of failure then hope your opponent is gonna make a grave mistake. Sure, you don't want to lose, but you should aim to win. If you want to improve, play matches with the mindset to improve. If you go to a tournament you should intend to win, so have the mindset that you will win while not letting it get to your head. If you're having trouble identifying your problems, you can also get real time advice on this website/ Start recording your matches on a camera or phone and upload them on to a thread here and ask for advice. The community is always willing to talk and give their opinion which is valuable because some people on this website actually know what they are talking about.
