Fanny must actually be pretty resilient to survive under the merciless bullying of Mrs Norris. Just one example, when she was about to go to dinner (for the very first time) at the parsonage, her aunt said to her "Remember, wherever you are, you must be the lowest and last." She was only saved by the kindness of Edmund and Sir Thomas, but even so she thought she was worth very little.
That sort of relentless putting down is sadly more common than we would like to think it is: apart from cases of individual bullying, we have the shameful history here of the treatment of indigenous Australians through most of the 20th century, and other countries have their own examples I expect. If you go on telling people they are stupid and ignorant and so on, it doesn't take long for them to believe it and be destroyed by it.
Another comment on the "darkness" of the book: Sir Thomas has been on his estates in Antigua sorting things out. The UK abolished the slave trade in 1807 after the long campaign of that hero, Wilberforce, but I looked up Antigua and Wikipedia stated that "all existing slaves were emancipated in 1834". So the Bertrams were the beneficiaries of slave labour.
This is just touched on in a conversation between Fanny and Edmund, where she said: "Did not you hear me ask him about the slave-trade last night?"
"I did - and was in hopes the question would be followed up by others. It would have pleased your uncle to be inquired of farther."
"And I longed to do it - but there was such a dead silence! ..."
Mansfield Park was written between 1811 and 1813, so someone like Jane Austen would have been very aware of all the discussion about slavery, and Wilberforce's long campaign to have that vile trade abolished.