I struggle with the exact same thing. I think it comes down to being comfortable with the fact that you stutter. If you stop hiding it, and pretending you DONT stutter, you will stop scanning ahead in conversations, planning out easier words to say prior to speaking and over analyzing every nuance of your disfluency. Walls will begin to break down and that spontaneity you're seeking will slowly come back. It's hard man. Sometimes I'm a lot better at it than others. My speech therapist and I have been working on mindfulness which has really helped. We all know the techniques it's just putting them into practice when the pressure is on is the hardest part. Being comfortable in your own skin is a huge first step.