Thanks everyone for the great ideas. I am goin to have to shop around for some mirrors and storage.

Does your sewing room have an outside wall with an area where electrical wires and plumbing wouldn't be in the way of adding a big window? We added windows to either side of one of our fireplaces because there weren't any. I felt like a mushroom until we added them.

Can you pull the sofa forward so that the back is making your fourth wall? Then you could reach the shelves you're going to put there.

Hobby Lobby did have some mirrors that were framed up like windows. I bought one for my utility room.

When we built our house, the contractor suggested adding hidden florescent lighting above the kitchen cabinets to cast light up towards the ceiling. I didn't think I would use this feature much, but it is my preferred lighting. We have north-facing windows, and in the summer with the trees fully leafed out, it can be very dark. I like to turn the florescents on as primary lighting instead of the can lights until I need task lighting for something like chopping vegetables. Now you can buy strips of LED lighting and do the same thing.

also that little 'nook' behind your treadmill....I'd build floor to ceiling shelving for added storage.

I commented earlier about giving the room to the boys and you taking over their rooms. That was before I saw the photos. I notice you have a glass door. That's is not bad. Is the sofa going to stay in the room and the treadmill? If not I would put the cutting table across from your sewing machine but scoot it back toward the area of the sofa. If you have a garage or can put the treadmill elsewhere I would. Where the treadmill is you could put a bookcase or shelving or stackable drawers. The area behind your sewing machine you could use as a design wall. Instead of putting a quilt (unless it's small) directly across to implicate a window, I would put shelving with a small space for a quilt to hang and shelving above, on both sides and below the small quilt. Where the sofa is absolutely put shelving. The wall between the sofa and treadmill You could put a mirror and one on the wall opposite or use that wall for pegboard to hang your rulers and templates and scissors an rotary cutters. Under your cutting table, you could put clear storage bins for other material.

Good lighting and cheery wall hangings go a long way...

Lowes and Home Depot both have posters that look like a garden window overlooking the ocean or some other beautiful landscape. they are about the size of a normal window and would give you that feeling of light coming in

If you are looking for storage and a mirror, why not get a nice bureau with a mirror; or, get a storage unit of some kind, put your cutting board or ironing board on the top, and then add a mirror on the wall above it. I would put rollers on it so you can move it. You could also use the stick-on mirror tiles. I would definitely add shelves to the wall above the sofa and behind the exercise equip. Consider the white melamine shelves because of the color.
Also, as a suggestion, you could put up the shelves but add the strip lighting, or the rope lighting. Either way, go for it, make it just 2 or more colors, even throw something over the dark sofa, like a quilt. That would take care of the dark color. If you put up a large shelf with a mirror under it, put up a daylight florescent under the shelf. Enjoy!