In colder portions of our house, we use an electric oil heater. It’s mobile and it heats the surrounding area and doesn’t seem to be as expensive. That’s a quick fix and you are generally not trying to heat the entire house, just the areas that you are occupying. Couple that with an electric blanket to directly heat you while sleeping.

Heat pumps and gas furnace like others have said are more long term and expensive fixes but worth it over time. It sounds like though you may need to assess where you are losing heat and see if you can shore up insulation and cracks since that will be a problem for any fix.