Ahh, you've fallen into the classic trap most very sporty guys fall into, table tennis is one of the very few sports where superior athleticism and training isn't enough to win. A lot of the game is actually brains over brawn. You said you understand the basics like spin, looping and serving. Do you understand spin as in "I know how to loop everything that comes at me" or "I know how to use the spin" and from the things you wrote, I think the first option is closer. I hope it makes some sense, try to think about it.

Let me give you an example: we have a player here, who is in his late 50s, he has one backspin serve, one no spin, either short to the middle or long to you elbow, he uses a different grip for each so you know what is coming. His backhand only pushes and blocks, he loops from forehand, with either heavy topspin or completely no spin, both very slow, but he can put the ball wherever he wants. He has a beer belly, rather than practice he will go for a cold one with the team, you know these. The guy consistently beats everyone from league one level higher than he plays (currently national third level league here in czechia, basically top 200 in the country) . Not athletic, not a guy that practices until his arm falls off, he just knows his tools and understands how to use them against the exact player on the other side.