Sasha Attwood said she has faced a barrage of abuse on social media.
She described it is “toxic” and it is “really sad” that teenagers as young as 13 are growing up thinking it is OK to send abuse online.
Ms Atwood, who made the comments in a video on her YouTube channel, said she keeps things private and off social media “because people are mean”.
In the video, Ms Attwood said: “This whole thing since it’s come out has just shown me that people are literally so mean.
“Genuinely, I was receiving, like, 200 death threats a day. I’m not even exaggerating when I say that.
“So many messages, every single day, and I still get them now, all day every day: ‘I hope you die’, ‘I hope you get cancer and die’, ‘I hope your whole family die’, ‘I hope the next time when you’re in the car you crash it and die’, ‘I hope after Wembley you die’.
“I never realised how bad it actually was, and the scary thing is it’s young girls.
“I’d go on these girls’ accounts who have sent me stuff and they’re literally like 13, 14, and it’s so sad.
“I try and put it down to age, but then I think I was never like that at that age. I never sent a message like that.
“I just think it’s so toxic, social media, and it’s really sad that these generations are growing up thinking that it’s OK to say things like that.”
Instagram has launched an investigation following Ms Attwood’s remarks.
The newspaper also said that TikTok confirmed it had removed comments highlighted and added that its community guidelines make clear this behaviour is not acceptable.
Grealish, who has been linked with a £100 million move to Manchester City, is currently back training with Villa.
He first met Sasha at school in Solihull. They recently spent a break in the Croatian city of Dubrovnik following Grealish’s role in helping England to the final of the Euros.
Ms Attwood’s revelations about the abuse she has had to endure on social media comes after some of England’s black players were sent racist messages that again raised questions about the way sites are policed.
Some Instagram users said they had reported abuse including slurs and monkey emojis to the site but had received replies saying it did not violate guidelines. It was reported Instagram and Twitter accounts behind racist abuse sent to England players following their defeat to Italy in the final were still active despite the sites being urged to crack down on discrimination online.