I also feel like I'm sometimes in disbelief about my conditions, despite being diagnosed and other doctors confirming them. I thinks it's a weird form of imposter syndrome

Yep!! I'm autistic (been diagnosed about 6 years), at home I accommodate myself, have unconditional acceptance, etc. so i really don't think about or notice that i'm different from anyone else, and then I start to wonder if I'm really autistic. It happens to a lot of us that we start to get imposter syndrome if we're not suffering 24/7. I imagine that over nine years, you've learned a lot about how to manage and accommodate your bipolar, which maybe makes it seem less intense or present, but it is still very much there :)

I feel this way with my BPD. Sometimes I think I'm just depressed but I show all clinical signs of having BPD.