Yep, happens all the time to me.
I find that its because,
1. Im quiet in general because when I get a little loud the tone of my voice makes everyone think im being agressive towards them.
2. NT's are used to just rambling crap and having to be louder than everyone else so they can keep rambling their crap, whether anyone is listening to them or not.
3. Quite a lot of people dont have the mental ability to hold a lengthy conversation and return to previous points, so they tend to reply almost immediately to whats being said, which naturally steers the conversation in other directions, including away from what you may have wanted to say.
As to having people talk 'down' to me or like im not there, yep, that definitely happens once people know about my diagnosis. Sometimes its handy, sometimes I let the NT's do everything for me because they offered because they think I can't do it myself. Sometimes its really irritating to have people talk for you about you, but its also an interesting way of gaining someone else's perspective of you.
Occasionally if im tired and in a mood and they start babying me I'll just stare them in the eyes, say 'ok then' and walk away.