The 32-year-old black and transgender activist was dropped by the cosmetics brand in 2017 after she spoke out against systematic racism in a Facebook post.
On Tuesday, L'Oreal Paris brand president Delphine Viguier said Bergdorf had agreed to serve as a consultant on the brand's UK Diversity and Inclusion Advisory Board following an "honest, transparent and vulnerable" conversation.
But Bergdorf took to Instagram on Wednesday to share the barrage of abusive direct messages she has received on the social media platform since her appointment, saying she has been targeted "every time something positive happens in my career."
"They can't stand it," she wrote, adding that some people see it as an "affront to their identity, so they work to sabotage whatever I'm doing and break me down mentally."
Bergdorf went on to state that she was not seeking sympathy but simply wanted her white followers to know the "cost of speaking out and being visible as black and trans people."
She added: "It isn't as simple as speaking up. It comes with a cost to our mental health and physical safety."
Her reconciliation with L'Oreal is the latest example of how protests sparked by the death of George Floyd are forcing companies to take a hard look at their policies. Floyd died after a police officer in Minneapolis, Minnesota used his knee to pin the unarmed black man's neck to the ground for nearly nine minutes.
In the lengthy Instagram post, Bergdorf told her 387,000 followers that she sought to highlight "why allyship is a necessary component of anti-racist work."
"White folk -- Please bear in mind that this is what many black activists and trans activists inboxes look like. This is why we need you to be doing the work also. This is why allyship is a necessary component of anti-racist work."
Bergdorf concluded her message on an optimist note, by quoting author, poet and activist Maya Angelou, writing: "But still, we rise."