How do I stop freezing up when I talk to people?

Ever since I was a kid, I’ve always had this **major** issue where I won’t be able to talk “normally” around those I am not comfortable with; sometimes, it will happen with those I am already comfortable with as well. And generally, I tend to either go completely quiet, or act like someone I’m not, by changing my personality to match the type of people I’m around (or, just have no idea what to say and start fumbling over my words).

It’s really starting to hurt my mental space because most of the time that this happens is when someone else is trying to talk to me about something serious (or, is having a confrontation with me), and I simply lose all thought. Then, after I leave the room and have my thoughts to myself, all the thoughts about what I wanted to say flood my mind and I get so mad at myself for just letting the other person say whatever they want without retaliating, even if it’s not necessarily a bad situation.

I believe the reason I do this is that I don’t want people to know my true personality, especially when the conversation is a bad one. I’m a very blunt & rude person when I have to be, but I really just want to keep the peace and want to stay neutral without getting the other person mad. When it comes down to people just trying to talk to me about something, the same can be said, but not because I am trying to not be rude (or hide my personality). It’s simply because I have ADHD and my mind is EVERYWHERE all the time and I have an incredibly hard time focusing on what to say, and what they had said.

HOW can I learn to relax myself and just be myself, without having a filter over everything I say in order to try and keep the peace? Though, obviously, sometimes I should keep the peace, I tend to over-do it and it’s really not good for me.