When we think about plot and antagonists, we tend to think of action/adventure stories: big revolutions against oppressive governments, a final stand against an evil vampire syndicate, completing an epic quest ending in a spectacular battle... but you don't need any of these things in order to have a plot or an antagonist.
Antagonist
The antagonist doesn't mean "the villain." The protagonist is the primary actor in the story. The antagonist works against this actor. Antagonists can be a who, but they can also be a what. Just a few examples of antagonists that aren't people: weather, bad luck, disease, personality, mental illness, rules or laws... really anything that makes it harder for the protagonist to reach their goal. In the case of your story, with an ex who made her feel like she had to conform to neurotypical behaviours, there's going to be a confidence issue that can serve as the antagonist. Even if she knows she doesn't need to conform, her ex will pop up in her mind as a little bubble saying that she does and that second-guessing of herself will create obstacles for her.
Plot
Remember, plot is really just the sequence of events that form the story. We like stories to have a bit of structure to them, so it's good to have a central idea that your story revolves around. What is the purpose of your story? What are you trying to say about your subject? In the case of your story, it sounds like the point you want to make is that it's okay to be different–that we should embrace our differences. So, how do we make this point in our story? We give our protagonists a goal that ultimately leads to making that point. If your protagonist's goal is to become part of a social group to gain acceptance, but they ultimately learn to accept their differences, that's basically your plot right there. All you have to do is figure out the details: what motivated this desire to find acceptance among this group of friends? What obstacles get in the protagonist's way as they try to reach that goal? What happens to make them realize it's okay to be different and to embrace those differences? Ultimately, your plot is the sequence of events that illustrate what your character does to try to reach their goal, what obstacles they come up against, and how they overcome (or don't overcome) those obstacles.
