I used to get this a LOT, Sarian... actually, I still do and from the same source - my mother.
I've never been sure why, but my mother in particular has an absolute need to make it seem that I am stupid/mad/evil/failed/unable/useless... the list could go on ad infinitum.
In her case, I think it stems from a very hard upbringing she had that has made her very hard (to the point of outright cruel) and incredibly dominant. SHE and no one but SHE rules the world in which she lives.
If she thinks that someone (not outside the family, but inside the family) intends to break free or "rise above her" in any/some way, she'll take the wind out of their sails big time and at any cost.
She also strives very hard to get others to see me (and others, when they "cross her", such as my father) in the same light she sees me, and those who trust or believe her tend to stand off me, in case it's true and I really am The Anti Christ
(one of the better names she has labelled me. Ah, sometimes, if only...
).
Yet, for anyone outside her family, she will come home bragging about their successes and achievements... especially if it is something she knows one of us have been working for. Go figure.
I think, no matter the noise they make and the dominant, aggressive stances they take, it's to do with their own weaknesses.
Some people make themselves feel better by picking a target they know will not truly retaliate against them (especially over a long term) and trying to keep that target lower than they themselves feel.
Maybe it stems from jealousy; maybe fear of being left behind; maybe because they think they will be found out as not being as good as a child/spouse.
But it is their weakness or problem. Not yours.
Even so, when you are on the end of it, it always feels like your problem and I can't really say much on coping with it other than (oddly enough) to write it down every time and start to see the pattern of it take shape; and each time, write down how it made you feel, and who you really are....
And get away from it as fast as you can. Try not to take part and "defend" yourself. You won't find some magical sentence that makes them stop, or makes others know the truth. Don't take part, or you end up acting and looking pretty much like they want you to - so "proving" their point.
And NEVER believe it of yourself. NEVER. EVER. No matter what.