Forma's advice is an excellent start. Watching replays can inform both of what you can do better, and also lets you see how the enemy overcame you.
Play objective, but play it smart. Don't count on your allies to play objective unless they show an inclination to do so. What you want to do is look for openings - kill 2 or 3 of the opponents and then get on tower or grab the rainmaker. Too many people will do those as soon as the match starts, inevitably dying and putting their team at a disadvantage. Being the best rainmaker carrier or clam thrower means nothing if it isn't done at the right time. More than anything, Splatoon 3 isn't about performing a successful push but sustaining that push, you want to kill the opponents and take the objective and then keep them from taking it back. Don't ever allow yourself to become complacent even when you have lead.
Consider your weapon choice, and maybe look at more conventional weapons. Yeah, this can get boring, but you'll likely have better odds going in with a splash-o-matic or other shooter as opposed to a more esoteric weapon class. Trying to rank up means playing to win after all, and you win by exploiting every advantage you can, for weapon practice and messing around there's turf and open. Of course if you have confidence in your skills, go forth, but generally speaking shooters and such are easier to handle in series compared to, say, a brella. This can also save you from the matchmaking - if you choose roller and the game makes your entire team rollers, good luck.
Positioning and aim. Spend time in recon studying where you can shoot from and where the enemy can hit you. Do aim drills (google that) in the lobby to improve your reaction times and accuracy. A lot of losses can come down to being in the wrong place or losing a crucial fight.
So yeah, Splatoon is a team-based game, but individuals can make a huge difference. If you're losing consistently then there's room for self-examination. I also recommend resources like Squid School, lots of good information there.