This is super common, but doesn't necessarily indicate that your stutter is a psychological problem. I used to be the same way in speech therapy in school. I'd go in wanting to stutter, so they could hear it and get rid of it, but then nada. I wasn't fearing the stutter, dreading the moment of disclosure, or even trying to be social or project any image. It's like being stripped down to barebones, as far as non-verbal communication goes. Then the more I morphed back into my usual demeanor, the stutter would appear again. It can be very easy for our brains to overcome a disfluent flaw when we are without stress, anxiety, dread, excitement, social fears, etc. We have a lot of energy left! But circumstances where we are without those emotions are few and far between, and we can't have any reasonable expectation that we can one day stop feeling anything and allot that extra energy for our brain to overcome a motor skill issue. Stick with the therapy, if you think it might help! SLPs are used to these things. They aren't doubting you or suspecting you're fluent.