We have 12 ft ceilings in our 1901 home. Odd fireplace and 4 doors!

I would get some plywood or luan and attach to the wall above the mantel. ( Width of fireplace x height of door frame ) Attach some molding around the edges. Paint it white. Rehang mirror.

I would also look for a narrow bench or chair to place to the side of the fireplace. Not so big as it would get in the way of the bedroom entrance but would encourage sitting by the fire and making a conversation area.

Go vertical! You can definitely add height to the fireplace by using tall lamps or candlesticks on the mantel on each side of your mirror. (See mine, below. I got them at Goodwill for $5 each, and the shades for $7 each new.) If it's not too high to see into, raise the mirror a few inches. You can add some detail above the mirror with decorative molding.

I don't know how much you plan to spend, so I'll start with the least expensive ideas first: Fake a window on an empty wall, and hang the curtains at least as high as your doors.

Paint: I know that it seems counter-intuitive, but right now, while the contrasting paint color below your chair rail molding looks great, it disproportionately cuts the walls in two, with the much shorter section at the bottom. I would suggest removing the molding and painting the walls all the same color, or just painting the lower walls the same color as the upper walls. Because your baseboards are 1x6's, it makes your wainscoting seem even shorter. You could raise the chair rails, too, but it would be easiest to paint everything the same color.

Add height with moldings above the doors and windows. You could also hang plants from the ceiling, use tall floor lamps, etc. Basically anything that will draw the eye upward will make the room look more proportional.

A tall bookcase, cabinet, or baker's rack, painted the same light color as your trim would look nice next to the doorway to your kitchen (opposite the couch), as well as providing space to display your treasures or pretty dishes. Something similar, which reaches about a foot from the ceiling, would really set off your fireplace, if you place it to the left of the fireplace, and if you elevate the mantle with tall accessories.

I hope this helps! Your home is beautiful, with those high ceilings and real wood trim and nice colors. You've just been following the 'rules' for shorter ceilings - like hanging pictures only at "eye level." I always wanted to know whose eye level, a midget or a giant?

Good luck!

To bring the ceiling down you could add substantial crown moulding and even painting or staining darker will make the ceiling lower. You can also paint the ceiling a darker color, or a metallic color, or distressed ceiling tiles. https://www.pinterest.com/search/pins/?q=how%20to%20make%20the%20ceiling%20appear%20lower&rs=typed&term_meta[]=how%20to%20make%20the%20ceiling%20appear%20lower%7Ctyped

Bigger pictures and decor hung higher would help a lot! I have 13 ft ceilings and hanging small single prints or pics just get lost. Try large groupings of small prints if that's all you have and don't be afraid to hang higher . A few large pieces will help you blend and draw the eye up.

You could use Faux Beams across to give interest!

Can you close off any of the 4 doors?

https://www.youtube.com/watch?v=6RbBmFNb9RU