Sounds like a stutter. In my opinion while you have a conversation with someone, as soon as you start stuttering, your brain focuses on the stutter. As a result, your focus goes away from the idea you were going to communicate. That makes you feel like you are having a thought block. I am not sure if you have already tried this, but you should practice pronouncing sound softly. For example, while pronouncing /r/ sound, start by 1) taking a deep breathe, 2) slowly breathe out without making any sound, 3) slowly roll your tongue like you do while saying /r/, 4) now produce sound from your vocal chord. At this point a person next to you should hear /r/. Do these steps as slowly as you can, for all the sounds that you find difficult to say. All the best!