It's not your fault, don't feel bad. The lives of fish are very fragile, and it always seems like they come and go so quickly. I hope that the betta has a peaceful passing, but you said yourself that you've done everything you could. Now you know not to use that plant again for your future bettas.

I'm not the best with words at times like this, but i'm deeply sorry that you're feeling bad and that your betta will be off to the RB