It all depends on the climate you're in. Where do you live?

Important is to apply insulation on the inside of the outside wall, create ventilation (can you drill a hole in the wall?) and add heating (electrical heating is the safest)

You could also consider running a pc with a graphics card that mines cryptocurrency. eg a 200 watt gpu heats your room and could also make you about 75 dollar per month that you can use to pay the heating bill.