I have had problems in this area as well. I find it hard to walk the fine line between being assertive and being a jerk sometimes. I found the following article which i linked at the bottom of this post, which i think sheds light on why it is so hard for aspies to not let ppl dominate them. I think this quote, where the author answers why she tolerated what should have been intolerable behavior toward her from peers, is it in a nutshell:
"Because I had learned to look to others for what was considered socially appropriate, even if it was painful or uncomfortable to me...My peers tolerated this behavior and thought it was OK, and I took their lead. This was the product of years of being indirectly taught to discount my own pain and discomfort in a given situation if it conflicted other peoples’ opinions of social propriety."
We are taught from the beginning that our instincts are wrong and that we have to look around us and mimic what we see, in order to function. It is hard to go from watching and mimicry to being assertive, especially when our desires conflict with social norms.
I am still working on this area. I watch social interactions very carefully as one standing back looking in. I look for the balance of power in the interaction--who holds the upper hand. It will be the person who is in control of himself/herself. Where both parties are in control, you have equal footing. This is where we can "agree to disagree" without fighting. I view this as a successful interaction for both parties.
Where one party can "push the other person's buttons," --make the other person mad, make them look stupid, etc.--the balance of power will be skewed and it is a short hop from there to one person dominating the interaction and the other person being stepped on. This is not an exhaustive explanation because there are more layers to it--but that is where i would start looking if you have not studied social interactions in this way before.
https://www.psychologytoday.com/blog/as ... ing-say-no