Unashamedly this happens to me. I really messed myself up meditating an hour a day for six months. It just became like a personal hell. Going into that room and turning off the lights somehow gave power to my fears and nightmares. I did however learn to let go of my ego a lot more, which I really enjoyed, and worked through a lot, but in the end it was deeply upsetting. I want to try it again sometime, but for now I've not gotten up the courage.
I think the issue is that we don't have meditation teachers. I've had issues with meditation as well, because I didn't know how to deal with certain challenges.
Nowadays, I know how to use meditation as a tool that purely benefits me. I use it sparingly, since I'm a bit lazy. But I've noticed that using it sparingly is erring on the safer side while still getting the benefits.
In my case, I notice the benefits after each session. My mind is calmer and I get less easily upset. In another meditation I'm more empathetic. In another meditation I'm more easily loving.
All effects last for about half a day to a day, but they have ripple effects (i.e. had a good day yesterday? Chances are your day will be great as well today).
Have you tried changing it up a bit? Pretty much any meditation instructions I've heard concentrated on what works for you. For example "you can close your eyes if you're ok with that", "find a comfortable position, you can lay down", etc. If going into a dark room doesn't feel right, you don't have to do that.
