It is not Mission Control OR Stage Manager. It is Mission Control AND Stage Manager.
Sounds like you just need to delete the extra Spaces you created in Mission Control. If you engage Mission Control via gesture such as a 3 finger swipe up or press the F3 Mission Control key then move your mouse to the top of the screen and hover over the extra Spaces thumbnails. An X will appear allowing you to destroy the Space. Rinse and repeat for each Space till you have only one. Then exit out of Mission Control and just use Stage Manager instead. But you can actually use both at the same time.