Why are interpersonal relationships out? I think rationality can help a great deal here.
Excellent article and topic. I suffer from this. My main problem (which is merely an excuse) is that there is a difference between what I think I want to do and what my body and mind actually wants to do when it's doing the things I tell it to do. Multiple selves become evident when this happens. The self that has planned the actions, and the self that - in doing those actions - gives up to do other (more fun) things. My approach is to make successive changes to the self who does those actions that 'I' plan, by trying to implement rules for him to follow. But I find it a constant uphill struggle, because he always outsmarts me.
I would say that being rational - as Robin defined it: more "rational" means better believing what is true, given one's limited info and analysis resources - might, but should not, and does not necessarily harm you.
How can rationality help you win? Maybe:
Better believing what is true, I don't think, by its self, necessarily must lead to a worse outcome then believing the falsehoods (in your example), it is just an example of where believing the falsehoods isn't outright defeated by the alternative strategy provided by 'rationality'. With this in mind I think the idea that rationality can help you in your interpersonal relationships is a very interesting path to follow, and should be followed.