My thinking voice in my head has gradually gotten quieter and quieter, and now it's like my mind is blank. i can't think clearly in a convo. i can barely visualize things anymore and it used to be vivid. what is causing this and how can i fix it?

Start with your Dr.: There is no simple answer to this. Thinking is such a complex process and the inner voice is not well understood. It could be that hormonal changes and neurological pathways are linked in such a way that maturation changes how we think. To begin with, see a regular Dr so that they can rule out conditions and meds that cause "brain fog." They can also refer you for next steps.