"As a forensic psychologist I can assure you that if a woman expresses herself in an assertive manner, she will most likely be recognised correctly as being assertive - not aggressive."
Don't try and pull the "I'm an expert, therefore I know more about this than you do" stunt on here. As a woman and a feminist, I can assure you that if a woman expresses herself in an assertive manner, she will very often be held to be aggressive.
There has been masses of research done on this. I can't remember who posted this ages ago, but there was some fascinating stuff about how in mixed groups, when women spoke for 35% of the time and men spoke for 65% of the time, the audience who was watching them, both men and women, agreed that the women had dominated the conversation.
As for a woman who is perceived to be aggressive when she's assertive:
Clare Short
Harriet Harman
Oona King
Linda McCartney (until she got cancer and then she was beatified)
Diane Abbot (until she became a national institution)
Heather Mills McCartney
That's just off the top of my head. I'm sure there are loads more but life is short.