How can I lighten my dark walls?

You can try 'washing' a lighter color over it, but it will give you a different look.

First, scrub the walls, thoroughly, then re-paint with a thinned coat of white, to lighten the grey, which will show through. Don't see any other way of lightening it, given the intensity of current grey.

You can try a large stencil making the wall look like wallpaper with a white design. Or Just paint the lower half of the wall. It will look like you have a chair rail.

All very good suggestions, above. If you really can’t stand it I would go with repainting. Sounds like the same amount of time will be spent either way.