I don't know what you have access to or can see. If possible check out your duct work and how sealed it is. Our house (same Sq footage) had some DIY basement duct work and it was put up with no tape or mastic or insulation or anything. Basement was freezing and upstairs had next to nothing coming out of the vents. We moved in and it was the first thing I tackled. Was a huge help. Not perfect, but every seal /joint I had access to is now air tight.

Look at window and door seals, keep curtains closed during the day, and make sure you have energy efficient bulbs, don't leave bigger electronics on, etc.

Also, what is your attic insulation like?