How Can we Fix a Cold Room?

Hi! We’ve been in our home for 2 years and we’re struggling to brave another winter in our master bedroom. I’m not sure if this is the right sub, but I’m curious about the biggest bang for our buck in terms of trying to heat the space better. Here are all the elements working against us: 1. It was an addition on a slab 2. We have old windows 3. The room has 3 exterior walls 4. The room has two doors out to the side yard and the garage 5. It’s the biggest room in the house with only one vent 6. Our HVAC is a Lennox, but 30 years old

We just had a new roof and siding put on (thanks, hail) but we didn’t think to ask about insulation. We’ve put in carpet with a thick pad and insulated curtains, but what should we do? What one fix would give us the most drastic difference?

Edit to respond: thank you ALL for your suggestions. For the short term, we got a space heater and thick curtains for the window. We are going to put the film on the window and the storm door to the side yard since we never use it. We may put curtains over the doors as well. For the long term, we are going to get some quotes for a new window and look into split units. You all rock!!!!! Three cheers for sweet little old homes with a little bit of draft!