How can I add ventilation to my room?
Did you replace your original wooden windows with plastic ones? Many old apt buildings relied on imperfections in wooden windows to provide natural ventilation. New windows are more air tight and  it probably makes the room stuffy.

Many casement windows have " micro ventilation" feature if you put the handle in a certain position. The leaves the window slightly ajar and provides enough fresh air.

Another option is to install a ventilation system.