We had the same problem, for us it was the design of the house and poor insulation. Our fix, a digital controlled radiating heater. They are ultra quiet and you get exact temp you want!! also you can keep the rest of your house at its normal temp and save you some $$. keeps the room at a great temp and we've had the same one for a fews years no prob.
something like this https://www.amazon.com/dp/B096FQYFBJ/ref=cm_sw_r_apan_glt_fabc_2351DCE4Q9FXY8A6FCEJ