You cannot help (see definition #6 here) being born that way.
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If a person is born of gloomy temperament they cannot help it. (Lord Chesterfield.)
My question is: they cannot help what?
I'm not a teacher and I'm not a native speaker of English.
"It" refers back to whatever was mentioned earlier. It simply means there is nothing I can do to change it.
I'm really pessimistic. I just can't help it.
He never gets anything down from the top shelf. He can't help it - he's only 4ft 6.
I drool whenever I see Johnny Depp. I can't help it. He's gorgeous!
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
They cannot stop themselves from being gloomy- they have to live with that aspect of their lives.