That happens to almost all of us who stutter. I once read that it's because when you talk to someone, the social part of your brain is activated, which doesn't happen when you talk to yourself. So it's not a psychological issue, nor is it a matter of energy or insecurities; it's simply biological. What happens is that your social part connects to your speech part, and the latter has weak connections, hence the stuttering. I recommend you research the topic because it's fascinating, and you'll get to know yourself better.