I also have conversations in my head. So does DH.
I am diagnosed as ADD/ADHD, but I wasn't diagnosed until I was 26. When I was a kid I got bored very easily, only instead of going, "I'm bored" I'd zone out into my own little world or just get up and do something else. I was always talking, doodling, humming, tapping or moving. My imagination was endless. I was always thinking. This lead to being very creative. I can draw, sing, knit, sew, paint, sculpt... if it has anything to do with creating, I love it.
As an adult, a lot of the hyperactivity eased up, but I still had the attention span of a gnat. With some things, I can hyper focus. Video games, very complex "fun" things, physically active things; these are all things I can hyper focus on. I can get lost for HOURS in a good book or sewing project. I usually don't break focus until my body starts getting stiff or tired, or someone actually breaks through the "wall" between me and the world around me. Other things, like talking on the phone, watching TV or even brewing a cup of tea doesn't hold my attention AT ALL. I flat out HATE watching TV. I have to apologize a lot to people I'm speaking on the phone with because I'll randomly zone out. It's not that they are boring me, I just... I dunno. It makes me feel so bad, but I can't help it, it just happens.
I also tend to interrupt people a lot, or just blurt things out. It's generally because I just thought of something that oftentimes has nothing to do with what we're all talking about. This has led to so many embarrassing situations I can't even begin, LOL. I'm a very funny person, I am really good at making people laugh and I have a lot of friends, I'm just awkward. Really really awkward sometimes.
I'm medicated now. I'm on Concerta. When I first started taking it, it made me tired. I'm not gonna lie, I thought for sure I'd lose weight and thought "YAY!!!!" but that didn't happen. It does help though. My husband can tell if I didn't take my meds that morning. Today I was late taking them, and he teased me because I reheated my tea 3 times before he finally reheated it for me a 4th time and sat it in front of me. LOL.
My mind doesn't stop thinking, even when I'm drifting off to sleep. I lay in bed and shake one foot (tapping it) until I fall asleep. It doesn't RACE or anything, I'm just always thinking.
ADD also makes me talk my head off too.

Sorry. LOL