Mission Control/Spaces problems with two monitors

i have a 2017 iMac running OS Ventura, with a second monitor. this problem is seriously effecting my workflow. i just can't do the things i need to do.

i have several Spaces set up on each monitor. when i move from one Space to another on one monitor, the other monitor does the same thing! it's like they're joined at the hip, but they're not mirrored. this makes things unbelievably hard. i never set it this way, and it's only on this user account, and it's only just happened. i've tried the usual things, logging off and on again, restarting the machine, and finally switching it off and on again. nothing works!

also, the second monitor has lost its menu bar (on all Spaces). this is not so important, but still, really annoying.

and why do the wallpapers on the different Spaces keep changing? i set them up to be all the same (a single colour), and they keep changing to some of the most garish of Apple's options.

can anyone help me, please? i'm desperate!

ps. i've just reset this machine to factory settings, and reinstalled everything, so please don't suggest that! :) and it's been running a few days since the reset, it wasn't there immediately, so it's not that.

thanks for reading.