I find that when I am nervous or talking to someone who is watching my face intently it gets worse. Like people who make a lot of direct eye contact and after they're done talking look at me to see my reaction, something about that makes me self-conscious and I freeze up. Also when I'm talking to people who I am nervous of what they think of me, such as a professor, or potential friend, I am more self-conscious because I don't want to say something that they might misconstrue as rude. My solution: I've found that when dealing with situations I find myself likely to stutter in, I slow down my speed at which I'm talking, put less stress on consonants (that's probably the biggest one), and try not to worry what people are thinking of me. Also I try to go to into stressful situations with a smile on my face and try out different voices. Not like accents or anything dramatic, but just different manners of speaking, such as changing the pitch of my voice, the speed at which I talk, and if I have a problem word I just say it over and over again until I can get through it fluidly in context. When you're alone next try talking and holding out the pitch of the word you're saying like singing style- then move the pitch up or down and continue talking as normal. I find this really interesting as you can move your voice around certain frequencies and change how you speak.