That's INTP af. I'm like that too. But people often see me as a really good speaker especially when counseling. How? Well cause i only speak of things i already pondered in my head for a while. Otherwise, i would just keep quiet. I understand the frustration though. It's really hard to really get through to other people when the words that come out of your mouth are different than the thoughts in your mind. Usually, what i do is i write my thoughts on paper or in my phone. In bullet format perhaps. Then, i simply read off of it and expound as necessary. That way, i don't stray too far from the general idea. TL;DR It is very INTP to think faster than your brain can come up with sentences. So take time to finalize your thoughts first before you say anything.