The concrete is sucking the heat away. Heat is drawn to cold. The slab has a lot of thermal mass, meaning it will take a lot of energy to change its temperature. It's likely a losing battle since you are fight the ground temp.

Insulating the slab is the best plan. Carpet is okay since it has foam under it to insulate it. Anything you can put down to keep the room air from contacting the slab will help. You may want to look at pallets. They are often free and you can raise the wood off the slab for an air space that could be insulated with expanded polystyrene, fiberglass or even sawdust or newspaper.

If you can find some concrete blankets, get 'em. They are great insulators that can be spread out on the floor or hung from the walls. We used several of them to insulate the north facing steel doors in a shop and it was almost tolerable in there.

Look for a used pellet stove. Mine works great. They are fan forced so they heat up an area quickly.