If anything, Catalan is a dialect of Valencian since Valencian precedes Catalan. Source: look it up.
Catalans are taught a different story in school, which makes most of them believe your statement. Source: I am from Catalonia and I was taught Valencian to be one of Catalan's dialects, until I met some friends from Valencia who educated me on the subject.
I had a quick look around, and most sources seem to disagree with you, with the first records of Valencian coming from circa 1350 [0] and Catalan starting around 300 years before [1].
If you have any authoritative sources to back that claim up, I'd be interested in seeing them.
[0]: https://en.wikipedia.org/wiki/Valencian_language#Authors_and... [1]: https://en.wikipedia.org/wiki/Catalan_language#Middle_Ages
Those are the best, especially on controversial subjects. You basically say, that first you learned in catalan, that valencian is just a catalan dialect - and then you was influenced by people from Valencia who told you otherwise and for some reasons you rather believed them.
The question is, why should anyone here also believe those friends of yours? It just sounds to me, you have been influenced by different ideologies - as this is a politically loaded subject, where people usually do not apply scientific standards to a discussion, but rather emotions.