Insulating my Room for the Winter?
Even many experts underestimate the volume of air exchange through tiny gaps, holes, slits etc. So generally too much unintended air exchange is often the biggest problem. Can be reduced by applying all weather silikon or acryl at the edge where  window frame and wall are touching, and between glass and frame. This should be done outside and inside and is not expensive at alI, does not change the building substance. If the window is not airtight between it's moving and still standing frame, also silikon can help, but a sheet of foil (kitchen) must be temporarily inserted in order to avoid sticking/glueing the window frames together until dry. This helps for rotating windows. Cave: the material needs a high temperature to dry out, ca. 15 Celsius or above. The door slit can be closed with old  clothes put at the edge every time the door is closed or with a special brush to be glued on the door. 
Air gaps can be detected with a cigarette or candle on windy days.
A cheap heating is a car heating seat cushion (car accessories), heating from legs to the back like in a car, plus a voltage supply of 12V. There are also 230V/120V heating mattress covers for the bed. And a low noise hair dryer next to the bed can warm up the bed in 1 minute or less with very low costs. These 3 electric heating systems are very comfortable, inexpensive and very fast, and save money and energy and the environment. They are scalable, nearly maintenance free and can be transfered to the next appartment with no installation work.