It sounds like you don't really believe that double-think is impossible; you just have belief in belief in the impossibility of double-think, because you think that belief would be a useful one to have.

As soon as you start "trying to instill a self-fulfilling prophecy", you're going down the same road as the people who say "I believe in God because I think it's useful to have a belief in God."

To be clear, if you're trying to make it impossible for yourself to double-think by planting that thought in your head, that may be a rational strategy. But don't try to convince yourself that it's impossible for other people to double-think just because you wish that were the case; reality is what it is, not what we would like it to be.