I Am Always Told That I Am Too Nice

I’ve been told that I’m too nice all the time and it isn’t good. My dad said to us, “Dont be nice all the time. That’s how you get taken advantage of. You gotta show you mean side sometimes.”
He says that mostly to me a lot because I’ve been taken advantage of so many times, I am too forgiving, too easy going and always trust so easily sometimes and it has to stop.
My older brother and sister (both adults) even told me I need to bring more assertive, open my mouth and speak up because my problem is that I don’t and I’m too nice to stand up for myself and call people out. I am so not too nice. Even my friends sometimes are annoyed by me being “too nice.”