I totally empathize with what you're going through. I'm sorry you have to go through this.

I have felt similarly in the past. I used to pay attention to every time a person I was talking to, would simply stop listening. I was hurt, I felt boring, unimportant than others because I thought they don't listen to me but do listen to each other.

I've learned few things since;
It's okay not to have everyone's attention, maybe it's not in their interest. and that's okay.
It doesn't have to mean anything other than the factual thing: I talk, he does not listen. it DOES NOT mean I'm worthless nor boring. boring is subjective anyways
Sometimes people won't REALLY listen when I don't really feel like talking to them, but rather talk out of social pressure. If I don't feel like it, I rather let it go. so regard it as they are reflecting to me what I actually feel on a subconscious level; not interested

** If a close friend would act like it; I would try to have a conversation with him in which I will say how it makes me feel when he does that, and ask if maybe it is something I have said. It is wonderful how honesty (in a respectful manner) works
** When somebody acts really impolite in a conversation, I would sometimes note his behavior.
If somebody would just turn his back in middle of conversation and walk, well.. I think there is nothing left to say, he 'said' everything.