I mostly just listen to country music, but I always worry about what people will think of me if they knew that I just listen to one genre. I always say to myself, “it's not normal to just listen to one genre/ this artist/band is too popular, why are you listening to them?" or things like that. My friends listen to metalcore bands like Motionless in White, and I'm not really into that kind of music. They think they are better than everyone else because they listen to underrated bands. Also, I always compare myself to other people. For example, if someone likes a really popular artist/band, I think to myself, “at least i dont listen to that!" I try to be open minded about other genres, and I want to listen to other genres, but I don't know where to start. Anyway, is it normal to constantly worry about what music I listen to?

Listen to what you like and don't apologise for it.
I have learned, throughout the course of this life, that people often put others down to make themselves feel like they are somehow better or distinguished from all the rest. Even those that claim to be tolerant and non-judgemental have a tendency to be complete assholes to people for not doing it their way.
Nobody with any sense would judge you on what music you like. I love older folk, rock & blues. I've had tons of comments about it, asking why I liked it when there's decent stuff about I could listen to, that it was so bad a burglar would leave it behind, etc. (this happened!) I was thought of as a bit strange, but nobody stopped talking to me over it.