Welcome to the world of covert stammering! Speech therapy sessions that I’ve attended try to encourage you to stammer rather than block, attempting to turn you into an overt stammerer rather than a covert one. Its a lot more difficult than it sounds if you’ve been blocking for years. My blocks aren’t as bad now. Most helpful technique i find is to slide into a difficult word, so for example instead of m-m-motorbike youd say mmmmotorbike.