I was questioning the universal applicability of my mental models because I predict the existence of a perfect model where everything is predictable and I want it. That led me to stumble on the heuristic I was looking for, one that makes my models self-consistent and self-predicting: the fact that I have mental models means that the holder of those models, which is me, must exist too. Also, the very act of accepting to go through that process will serve to greatly thicken the branch of the trees of possibilities of each of our separate models predicting "my partner also has a similar mental model as me, at least with regards to my prediction of us being happy together in the future," which is what made me begin this whole statement in the first place.