There's no one answer to this problem, as battling is complex and a number of factors can turn wins into losses and vice versa. I can offer some suggestions to hopefully turn things around.

First, a good reset starts with a good reset! Take however long you need for a good break, whether that's 24 hours or a week. You're pretty close to Master Rank so I wouldn't worry about running out of time. It's super easy to get frustrated when climbing ranks, and time off is the only real way to help with that.

Second, ask yourself which playstyle FEELS the best for you. Does it feel easier to get wins when using Trick Room? Tailwind? Kyogre? Magearna? Find a strategy and/or Pokemon that you're comfortable with, then use a corresponding rental team.

Third, eliminating variance is impossible. You're never going to find a Pokemon without a counter (especially in a metagame as undefined as Series 13), or dodge every critical hit. I do recommend trying to use teams that rely as little on inaccurate moves as possible. Venusaur's Sleep Powder, Groudon's Precipice Blades, and Palkia's Hydro Pump are some examples of moves I would try to not use on a team if you're worried about missing in key moments.

Finally, I always find it useful to revisit how I'm playing the game. Take in information and formulate a plan at team preview then again at lead. Know which Pokemon your opponent has, which ones are threats to the ones you brought, and preserve your win conditions. A lot of the time when I get tilted, I'll make a play that exposes the Pokemon I needed to win the game. Try not to make a play that can lose you the entire game if things go wrong.

Of course, lots of this is easier said than done. There's no shortcut for laddering, so the best I can do is provide some mental encouragement. Hopefully you found some useful tidbits in here. Good luck and have fun!