You’re not screwed, but you might have to be a little patient. You can’t downgrade an Xbox’s OS, so if the most recent OSU1 isn’t newer than the OS version on the Xbox, it won’t be successful in updating. It sounds like that’s the case for you, so you’ll have to wait until Microsoft updates OSU1.
As for removing your xbox from the insider’s program, you’ll have to unenroll from your account. Once the mainstream update surpasses the insider build, your Xbox will update and you will no longer be on an insider’s build.