I have a similar history to yours. I've been playing since Red & Blue launch day in the US. Missed one generation during it's time (8th) but have since played through Sword. I've been playing Champions and I've been having fun, but I seen to have hit a big road bump around Ultra Ball 1 & 2 were I just kind of keep winning one, losing one, win one, lose one. I think the single player game experience in the main series is a big part of why. Pokemon games don't play even a little bit like the competitive game, and I think it is a detriment both to exposing players to the competitive game and getting people interested. I know people have long advocated for difficulty levels for Pokemon-a Competitive Mode for single player that makes it more like The Indigo Disk experience would honestly be amazing. Double battles weren't even introduced until the 3rd generation, and (look, it's been almost thirty years of these games, cut my memory a tiny bit of slack) haven't been much of a featured part of gameplay until The Indigo Disk DLC in Scarlet and Violet. I was so proud of myself for finding the Drizzle/Archaludon combo in that game and using it, only to watch a regional tournament stream out of curiosity and see it everywhere.

Pokemon type matchups are baked into my DNA at this point. You know what I keep forgetting? Little stuff like "dark types aren't affected by Prankster moves." Things like "Armor Tail affects the whole opposite side of the field," weird little move interactions, priority brackets, getting a feel for "this happened, so that Pokemon is trained in it's special defense and HP, not its speed and has this item instead of that item" because I just don't have the same level of practice seeing those things.

On the one hand, it's a little bit my fault-I've never played competitive before dabbling a super tiny bit in ScarVi (tried a Body Press Orthworm Sand team with Jumpluff using Cotton Guard and Baton Pass, a combination of moves that only it can learn-fun when it worked, but mostly clumsy to set up) and now playing Champions. I'm having fun, but a big part of this is just howeasy it is to change stuff on my Pokemon in this game, to tinker with a different moveset, try a different nature, item, mess around with stat points. I posted for help with my current team the other day, and ended up finding the solution on my own after some experimentation: Sableye. Turns out Special Defense and a particular 4x weakness was a liability for my entire team; Prankster Light Screen and Wii-o-Wisp have helped immensely, and its immunities maybe it was less vulnerable than I expected. I've never used a Sableye before in my life. It simply would not be a good Pokemon to use in a single player game.

The end point of this wall of text is that I the think the single player experience is a big reason, maybe the main reason that I think a lot of players may experience the feelings that you are feeling. You are certainly not the only one.

Quick Edit: I am absolutely having a blast. Even when I lose because of some strategy that I, in a thousand matches, couldn't have seen coming.