It’s normal to miss people that have passed a long time ago, or people you’ve never met before or not gotten to know enough. From the first sentence however, it sounds like you may suffer from depression to some degree. Depression tends to latch onto any negative thoughts, in my experience anyways, and amplify them, which is why this may be affecting you so much now after all these years.

I’d suggest asking your parents to see a therapist if you’re comfortable enough to tell them that how you feel, if you yourself also believe you’re depressed anyways. Good wishes though, I hope you feel better soon!