I've often had people talk over me and interrupt me. It's getting a bit better now, but it is only recently that things have really improved.
My take (in which I'm generalising for the sake of simplicity - I know it's not that clear-cut):
To me, it is natural to communicate in order to obtain information or a solution to something. In aid of that, if I'm uncertain or insecure it would be natural for me to show it because I want to get the facts straight, get to the real truth or to a workable solution. Obscuring things doesn't seem helpful to me in that setting - or it wouldn't if everyone were operating similarly to me.
However, many NTs only use communication about any topic not (only) for that topic but as a tool in vying for rank. Even when there's another goal (getting facts straight, finding a solution), vying for rank is typically part of the game of communicating. Powerplay runs through almost all interactions for many NTs. It's so natural that I don't think they're all even still consciously aware of it. It's just second nature. It's not all about actually being right, it's about convincing other's that you are (unless the consequences are too severe). It's not about being the leader in a given situation because you know best, it's about convincing others that you know best (unless the consequences are too severe).
For many NTs, there seems to be a middle ground between these two objectives: the seeming objective of any unit of communication (the topic) and the powerplay. The best outcome is somewhere in that middleground - where precisely will depend on which of their interests predominates: truth/solution or rank. But it's not usually all the way at the truth/solution end of the spectrum, where I think autists tend to find themselves.
And and that's why I believe many interactions are subtle battles - for rank. It's not just talking over people, it's also tone of voice, pitch, posture, facial expression and so on.
In the past, I've found it very exhausting to just 'be more assertive'. Being more assertive without awareness of what else is going on will mostly just run you ragged, in my opinion. And trying to play an NT game for senseless rank is also crazy-making, exhausting and feels utterly pointless. So what's the solution?
To me, it's being aware that this is going on, first of all. In a second step, you can pick your battles depending on how cooperative the people you are dealing with are and how strongly you feel about the ostensible topic at hand. If you can take it or leave it, and if it isn't important that people see you in a certain way, don't expend your energy. Though, if people are being cooperative, conserving some energy shouldn't be too difficult because they won't be too inclined to talk over you and interrupt you and dismiss you. But if they do all that and you don't feel strongly about the topic, you can try to remove yourself from the situation.
If I can't remove myself from the situation (i.e. at work) or if I feel strongly about the topic at hand, I don't try to tap directly into assertiveness mode anymore because that turned out to be too exhausting. I try to make myself as acutely aware as possible of the underlying powerplay first. Then I think about what I want (facts/truth/solution and/or respect) and let myself really feel it. If I can see the powerplay for what it is - not not as seemingly inexplicable irrationality on someone's part - it automatically makes me more resolved to get heard or get the respect I deserve because I'm not a second-class person who has to take a backseat to someone else's ego.
When I can detect the powerplay in interactions, it's much easier for me to become assertive authentically (because I can see that they're trying to con me with insufficient knowledge and/or are trying to position themselves as the worthier person) than when I get confused by someone's seeming irrationality or self-importance and have to dredge up assertiveness in a vacuum. And being authentic is less exhausting than acting out a part without the corresponding thoughts and feelings behind it.
