Why are duels so hard at the beginning of the season? I need help.

Hello, I recently returned to the game. I reached Diamond rank last season with a Hagrid Dragon deck. I also have a Bellatrix deck and a Newt deck. I was progressing really well. Since the beginning of the new season, I’ve been losing all my duels. I’m currently at Grimoire level 63. I think I’m missing some cards. Do you have any advice?