You can only insulate from underneath. If you can get under floor in crawl space or basement you can lay in insulation. To repaint, sand and scrape off any loose paint, wash down with TSP. Prime with oil based primer, lightly sand then apply 2 coats of oil based paint. Be sure to use oil for the most durability and cleanability. Latex over old oil based paint will peel.
If you can't insulate, an area rug will help greatly. Good luck. I love my painted floors.
I have lots of painted floors and while oil based paint is desirable and I initially used that, the current oil based paint makes me ill if I use it inside. You can use latex paint over oil, but not oil over latex. I have done this when I needed to touch up my painted floors. For these floors I painted entire floor (this is in two rooms) white, then drew on pencil lines for alternate brown color. I used one coat for each color, of course the brown is over the white paint. Btw these floors have been done over 30 years ago and have not been totally repainted in all that time. Just touch ups.
Buy a good wood varnish, for in doors.
The “insulation” is for what? Thermal or sound?
Thermal requires getting under the floor, as in the Crawlspace or basement and addressing it from the bottom.
if you have slab-on-grade then it has to be removed (yes, really) and then an insulative membrane installed.