Cynthia Crawford, from Worcester, has not seen the film The Iron Lady, starring Meryl Streep, and said she could find it "quite distressing".
The opening scenes depict Lady Thatcher as an elderly woman who cannot remember whether her husband Dennis is alive.
The film is due to be released in the UK in January.
Ms Crawford has worked with Lady Thatcher since 10 September 1978 and still visits her regularly.
She said she was in a dilemma about whether to see the film which depicts Lady Thatcher with dementia looking back on her life.
"Because Lady T and I are great chums and friends and mates, and I have travelled all over the world with her, I think it's going to be very hard to see that," she told BBC Hereford and Worcester in an interview on Saturday.
"Her executors were given the initial script. I don't know if it's been modified, but we didn't care for the script at all. The children were very upset."
Ms Crawford has received offers of private viewings of the film, but said she was waiting for the right time.
"I expect eventually I will pluck up the courage to see it because everyone is talking about it," she said.
"I've had nothing but phone calls from the press... inviting me to watch a private view with [Lord] Heseltine, Norman Lamont and Nigel Lawson."
Ms Crawford features in the film and is portrayed by actress Angela Curran.
She said Meryl Streep had put "her heart and soul" into playing Lady Thatcher and had carried out "great research".
But she said: "I don't think it could ever be really depicted in the proper way.
"This is probably as good as it's going to get. I just think this retrospective angle is not happy for her or her close friends."