Invalidator

[Merna]

An [invalidator] is any person who is, for whatever reason, prone to being jealous and insecure. In order to think well of themselves, they will say and do whatever they can to harm the target of their jealousies and insecurities, and to discredit them in any way they can. This gives them a false sense of self-worth and superiority. They are all about bringing out the worst in others, and they will use any [unscrupulous], malicious, or manipulative tactic in order to achieve this end.
The best ways to confront an invalidator may include simply ignoring them, proving their accusations to be false, or to respond to their [negativity] with kindness or humor.

[Donald Trump]

Someone who generalizes [what you said] specifically to dilute its specific power to [render] your statement [useless].