I used to have this and it was as inconvenient as you said here. I tried to work on extending this 'skill' (really important to first acknowledge that it's not necessarily a weakness) by just feedbacking "reality-check" to scenario-modelling. For example in my "Deliver a class" mental scenario, I used to assume that I'd be as smooth as Feynman. In real life presenting, that's obviously not the case! I used to cringe and get frustrated so much, until I told myself to back off and let the process continue (indeed, it's not easy to shut your brain down!) So I tried introspection. I realise several factors about myself and what things from the environment affect me ... so I loop all this back to my mind so the next time I do another mental presentation scenario, I take these into account. It gradually gets more 'accurate' (by this I mean real outcomes don't seem as widely off-the-mark ... it also helps to acknowledge a large margin of error!) and now I use it for "important conversations" as a way to find key points/ questions and, surprisingly, to get myself more relaxed.

Hope this helps somehow!