How do you know when you're doing too much or too little assertiveness? Do you have an internal sense of that?

Kind of, yes. If I get really emotional, and teary-eyed and want to run away, I've come to understand that this is a conditioning that has me betray my own needs for the sake of others and that I need to fight through it. When I go against the urge to people please and instead try to make room for myself, it feels wrong, but I know now that fighting that wrongness is what will lead me to a more honest existence. But, even after years of practice speaking up for myself, and trying to set boundaries that are healthy and suitable to my well-being, it's still difficult to say, "No, I don't want that" or "I'd be more comfortable with X". I constantly question my own motives, my vocabulary, my freaking body language, all of it, trying to bend over backwards to not be 'too loud', 'too assertive', and/or 'too demanding' and it is exhausting. The truth is, for myself, I'm so aware (and wary) of people around me that I'm already not going to go out of my way to cause problems unless I am being outright abused. Knowing this, it's easier to address the smaller things, like saying no to added work or navigating social situations where I can just exist without having to constantly worry about everyone around me. You can't control others through your action or inaction. Try not to let the fear of other's emotions dictate how you navigate your own.

Ask yourself this: Is what I'm doing really too much? Am I really demanding something that is unreasonable? Think of a person who you find is too assertive. What don't you like about the way they go about it? What do you see them doing that gets their needs met that doesn't piss everyone off? If you pay attention, which I'm sure you already are, you'll learn ways to establish boundaries without sacrificing so much of yourself. It takes practice, and will be different for everyone. It has gotten a little easier for me, but there will always be times when it's more difficult than others.

The way I sense it is by looking at the person for signs of pain or discomfort when I talk. So I hold back when I know what I'm saying might be something they don't want to hear. I'm afraid that asserting myself will hurt people.

I feel this so much. Unfortunately, this is a reality that needs to be addressed and accepted. Odds are that you're gonna freak people out when you actually speak up for yourself the first time, if there is an established relationship like with family members or coworkers. I've had a lot of therapy, read a LOT of psych and self care books/articles/blogs (shout out to Devon Price's Unmasking Autism and other ND communities), and I've come to realize that my body tells me a lot about where I am emotionally, even when I've not fully processed what's going on in my head. Exhaustion, nervous energy, sweating, needing to stim; all of these things are usually indicators that something is off and needs to be addressed. It's not always immediate, but I've found that bringing up something that happened in the past can be just as effective as addressing it in the moment if it is a recurring thing.

The first time I actually asked for something I needed, I started crying. As in, full-on tears streaming down my cheeks, anxiety-attack-around-the-corner shaking and freaking out. I was terrified in that moment that I was asking too much, that I was being a burden, that I was making the other person uncomfortable, and that I was somehow wrong for even bringing up my feelings that had, up until then, been trampled and manipulated by others who were completely oblivious to my struggle because I never let them see the real me, and who were perfectly satisfied by my people-pleasing, codependent mask. To say it was jarring is an understatement, and I used the template Zuzak mentioned to do it ( "This is how your actions make me feel, do with this information as you will"). The response was...not great. The person I was talking to seemed to have an internal system crash error that caused them to react with anger and defensiveness, because how dare I call them out of their established patterns with me and make them self-reflect on their own behavior. I was accused of a lot of things I wasn't (selfish, rude, blah blah blah) and that my "boundaries were making others uncomfortable". I type that now and can't help but laugh/cry at the absurdity of it. What a horrific response, to have someone who will only accept you if you fit their specific mold, do what they say when they say it, and never question their motives. How dare I act like an individual person with needs and wants and feelings!

That being said, keep in mind that when we do stand up for ourselves, and when we ask things of the world around us, the response isn't always going to be pleasant, especially when there is a list of unspoken rules that we're trying to navigate. (You can see this mirrored in the US with our rights being taken away--the problem goes all the way to the top.) Facing this is a really hard wake-up call, and I've come to accept the fact that not everyone will like me. And that's okay, as long as I'm being treated with respect and not being forced to do things that make me want to delete myself from existence. I have been able to apply this thinking to several jobs I've had, where I had to say no even if it meant not being on the Boss' 'Perfect Employees' list, but I always did my job and and did it well even when I was going through burnout and depression and just trying to survive. In a work environment I would suggest more caution, since work culture is more of a cult culture than an actual working environment where things get done, but you don't need to sacrifice your own comfort for the sake of others. It's important to remember that everyone is human, including you, and more than likely you aren't the only one who is feeling a certain way in such an environment.