Luckily, I've never come across anyone who truly believed that dyslexia doesn't exist... but then perhaps those people have kept those opinions to themselves as they know that upon expressing those views, I'd bite their head off...
I feel that a lot of people don't understand dyslexia - teachers, in particular. When my brother wasn't performing well when learning to read and write in reception, mum realised that he was dyslexic. The teachers brushed away this explanation for his poor literacy and explained that he was 'an August baby' and how 'boys always progress slower' and simply wouldn't even acknowledge that he may be dyslexic.
After battling for years with the school and receiving no help, mum eventually paid for a diagnostic dyslexic test to prove to the school that he was dyslexic. However, even with the official proof that my brother was dyslexic, the school still wouldn't do anything. My mum then spent a lot of money on lessons given by the Dyslexia Institute (I think it now has a different name) and they really helped his literacy.
Dyslexia isn't taken seriously. The undiagnosed dyslexics are the kids who end up acting up in class because they never get any help, and they are then branded as the disruptive, thick kids. The dyslexics who behave (like my brother) and will sit quietly while struggling over their work are simply forgotten about and allowed to fail.
If only dyslexia affected your exterior - if the disability was visible, everyone would take it seriously and no one would dismiss it as 'not being a real disability'.