Being unable to control what you say might just be because of habits due to your reality checks.

Really, I think the problem here is that because of past experiences, the dream automatically destabilizes when you speak. You constantly keep expecting the dream to fade away after you say this, even if you don't mean to. I'd suggest trying to implant the idea that the dream will not destabilize into your subconscious - it might help with this.

Destabilization does happen with beginners, though - it slowly starts to happen less and less, so don't worry.