How do I be a "good girl" while still standing up for myself at the right times?

I don't give a lot to other people because I'm lazy, unintelligent, and insecure. So, I just try to be someone who's willing to do things for others and help them, while still staying in my place as a less useful person.

But, sometimes, it's just not worth it. Like if a guy is aggressively asking for nudes. In most cases, I don't know when I shouldn't allow someone to treat me a certain way. So, how can I know?

How do I be a "good girl" while still standing up for myself at the right times?
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