How to repair and improve my wall

Look up Stress Crack Tape on google. It's gotten good reviews for rather minimal work. Once it's on and blended to the wall with a bit of spackle and sanding, you can paint any color that brightens your life.

As much as I love gloss or semi, they do highlight less than perfect walls.

Depending on what your wall is made of - the repairs will be a little different. Google it depending on whether it is sheet rock, plaster, or perhaps something else.

If your walls are not textured, why not just use a wee bit of drywall compound to fill in the cracks? With the drywall compound, you don't even need to sand. Just smooth on the compound and let it dry. Once dry, run over it gently with a damp sponge and voila! you are now ready to prime and paint and will not see any remnant of the crack. I did this on my plaster walls that had settling cracks and you'd never know that they were there unless I told you. :)

If your walls are textured or swirled, apply the compound and use either a wallpaper brush to emulate the swirl pattern or a brush to recreate whatever the texture of your walls are.

Good luck and happy filling!