That's very accurate. When we are too nice, we risk that people take advantage of us. When we are too aggressive, we push people away. But if we're direct and are respectful to people at the same time, we can get much more done together.

Unfortunately, you only become more assertive by being more assertive. You must practice. What helps is to see every social situation as a negotiation.

I highly recommend reading Never Split The Difference by Chris Voss. In fact, I learned the "How am I supposed to do that?" technique from that book. When people give you a proposal you don't like, ask them that question. In my case, I changed it to " What am I supposed to do?"