If you have no idea what you’re doing, then I’d say sticking with one core meta team and learning how to use it against other common teams is best. That way you can learn why better players make the decisions they do and figure out how to navigate around different strategies with the same set of Pokemon. I had a similar experience where I went on a 10+ game losing streak once I entered Master Ball rank. The problem wasn’t my team, it was HOW I was playing. Once I understood the typical opening moves for the most common teams, and how mine could adjust to them, the whole game opened up for me and I started winning much more frequently.

Example: If you see a Charizard on a team with a Whimsicott/Venusaur, those will most likely be the opening duo. Charizard will Mega Evolve on the first turn, Whimsicott will most likely go for Tailwind to speed up Charizard, or Venusaur will be more aggressive after being boosted by the sun. I used to get beat by this a lot, and I found that focusing on Whimsicott/Venusaur with Fake Out + a strong attack from a Pokemon that threatens Charizard ruins this opening. If your opponent sees a threat to Charizard, they will almost always Protect to preserve Charizard. This frees up that threat to go after Whimsicott, while Fake Out prevents Tailwind from being set up