I do believe that she was coerced into having an abortion, but not necessarily just by Justin.
He may have said he didn't want to be a dad at 19, and that's fair enough to be honest. He was on the cusp of a huge career, as was she, and they wouldn't have been able to be good parents. Openly admitting and realising that isn't actually a bad thing. His behaviour to her after their split however....as a PP said, for 20+ years he's continued to piggy back on her fame and slam her every chance he got (such as saying Vegas residencies was for has-beens, saying there's always been Madonna wannabes, and he's dated some of them...the videos for CMAR and WGACA).
But there's also the influence of her parents. Her father had been openly abusive her whole life. When Jamie Lynn was pregnant, he wanted to send her away and make her give the child up for adoption but JL had to fight for herself. Her mother openly said that Britney had no capacity or understanding of life when she got married in Vegas and forced Brit to annul the marriage. It might have been for the right reasons, but her parents have seemingly spent her life telling her that she didn't understand things, which I assume is so they can keep her under their control.
Then you've got the record company/management. Firstly, there's Brit's image as virginal child star - having a child as a teenager out of wedlock would have knocked that idea far away. Then, there's the juxtaposition of placing Brit as the sexiest woman in the world - let's face it, a teen/young pregnancy isn't sexy. They wouldn't have wanted that. Let alone the prospect of not being able to work their cash cow to death.
So I fully believe that while she wanted the baby, she would have been under extreme pressure from literally every single person in her life to have an abortion because it made "their lives" easier.