It's hard to not internalize the idea that you're uniquely broken and will never be able to experience the world in the ways that other people can. And while we all do have our unique difficulties, traumas, and flaws, in most cases we're in good company.
Therapists, perhaps more than any other demographic, are deeply familiar with just how many people think they're singularly damaged, and the converse fact that many of us struggle with similar hurdles.
I'd say a common one is believing that there's something innately, irreparably wrong with them that makes them unable to ever truly 'fit in'. For a lot of people it's such a deeply ingrained belief that it can be extremely painful to acknowledge or express, regardless of the level of personal success in their lives.
Two topics come up with regularity: when someone discloses to me that they were sexually abused as a kid, and/or when some is experiencing suicidal ideation. Both are something I hear from clients every single day, and so I don’t find it weird at all. But, when I have someone in front of me who’s talking about it for the first time, I know it’s important to validate the fact that even though I might be talking about this for like the fifth time that day, they have never talked about this EVER, and are in need of gentle care to feel safe.