Catching edges on flat stretches?

Hi, relatively new snowboarder. I have a hard time with long narrow stretches of trails that are mostly flat. They require me to keep the board straight, because if I slow down/make wide turns, I lose momentum and have to get stuck and unstrap/ walk.

However, when the board is straight/flat, I gain speed and catch an edge and fall hard. I can’t tell what edge I’m SUPPOSED to be on when the board is straight/flat (it’s like it’s switching heel and toe side without me realizing), so when I pick the “wrong side” to slow down, I catch and fall.

I’ve been trying to make open turns on these flat stretches so I can DECIDE what edge I’m on, and not let the board decide for me. But these falls are HARD. Any other advice?