Logically, I know it's true that demeaning and comparing other people's experiences is wrong, but I encounter it so much it's hard to really believe it.

I can relate to some degree, though in my case it was my parents. The good news is that it doesn't take much practice until you start noticing the difference.

There's not a lot of advice I can give you, because above all you need to figure out yourself what you want (and how you want to get there). Just keep reminding yourself every now and then that the most important person in your life is you, and anybody trying to tell you otherwise is unlikely to have your best interests in mind.