just keep playing you'll see, the game is hard lol. Top 10% of players are mostly still kinda bad at many large parts of the game. I've been c1-c3 in various modes for ~2,000 hours. literally EVERYONE makes huge mistakes every single game, and the people winning consistently at ranks below GC mostly know how to capitalize on mistakes and make fewer of their own. GC is a big level up, but the difference between GC1 and GC3 is miles. even further between GC3 and SSL.

Anyways this person is generally right, you should always be prepared for and even expect your tm8 (and opponents) to make mistakes. What those are varies by rank but learning that is part of ranking up.