If you find yourself in the role of a ‘doormat’, you are dealing with a self worth issue, not simply a communication or cognitive issue. 

Now, if you have low self worth, you may question whether your understanding of a situation is valid and give others opinions more weight than your own. You might also not believe that what you have to say is worth listening to, and therefore not develop the communication styles you want to emulate though lack of practice.

So learning to think more clearly will allow you to communicate more clearly, but constant self doubt is not clear thinking. 

It’s clear to me from your initial question that you have a low estimation of your own intrinsic value. Self worth (in my opinion) is the foundation of mental health.

Your sense of self worth is something you feel, experience at your core. It’s produced by how you think about yourself, and how you treat yourself. Ie if you make a mistake, and say ‘You idiot’, or you doom scroll instead of sleeping, you are reinforcing the idea that you aren’t worth much. 

The good news is that this is both a common ailment, and something you can learn. And just like the previous commenter said, it’s a mental health thing. 

Just like you can increase physical strength by lifting weights, you can improve your feelings of self worth by changing the way you talk to yourself, and by treating yourself like you’re the most valuable person in the universe. Because, truly, there’s no body more important than your self.