This is something a good SLP can help you with. Those faces are learned secondary behaviors, likely because at some points you were struggling to get a word out did some kind of physical contort and it worked in that moment. And the more that happened, the more reinforced it became until it becomes a new habit. I would start by being aware of all the small contortions you do and then work on reducing those. Giving yourself time and not trying to force out words can help. But you need to start small, because you likely won't be able to do anything during the big blocks or stutters