How can I remove decoupage that I already sealed with polyurethane

Uh, a blowtorch? Try full strength paint thinner. If you end up with a gummy mess, at least that, you can scrape. You also could sand with a coarse grade grit to get the poly off, but I think you would have to use an electric sander. Best wishes ☺️

ok so the above not so sure....and not sure if this will help you. http://lifestyleforreallife.com/2014/06/10/how-to-remove-mod-podge/

Use a paint remover first and scrape it. After that, sand it.

I would sand off everything and start from scratch.

You Need a stripper that will remove the poly coat. It will in turn loosen the decapodge. Prepare for scraping!! Then Sand lightly till smooth and start recreating 😊