You have a cozy space, with two windows that can be useful in providing natural light. You expressed concern mostly with the color of your room and the “cave”-like feeling created in your space.
To address your color issue, the biggest step is painting the ceiling back to white. Dark ceilings are not frequently successful, and when they are- it is on high ceilings. With a low ceiling and dark colors, your room will automatically feel darker and smaller. As an alternative, we suggest Behr’s Polar Bear paint for the ceiling, trim and doors. These should all be light colors to add contrast to the focal color on the walls- in this case, we’re suggesting Behr’s Herbal Mist. The light green color will help create the calming, cool effect you’re looking for.
Use this design to create your furniture arrangement and as a guide - you might already own some similar pieces or be able to find things in your favorite store at a better price. That's the fun of it. Your style, your budget and on your schedule.