I still block sometimes but what helps me is trying to have a peaceful mind. The reason I (like a lot of others) stutter is because we're super self conscious about our speech and our mind goes a 150 miles an hour. So idk if it makes much sense but try meditation. When speaking to people you can feel more comfortable and at leave that way you're not thinking abt certain words you might block on. The reason you don't stutter when alone is because you're comfortable then. No one can judge you when alone so try some techniques to makes yourself more comfortable when talking to other people. It's extremely hard at 1st but once you can do this and your mind is at peace you'll see great improvement.