Disruption in your Your breathing pattern causes speech blocks. The need to get it out and subsequent rush/anxiety makes it worse. First, record yourself speaking in a normal situation when you're not blocking. Observe your pace, pitch, pauses, and the general demeanor. Now try to be in the same state of mind in other situations when you block. This is easier said than done, and requires lot of practice. DM me if you'd like to talk abou this approach. It helped me get over my blocks.