" Money is supposed to be fungible, but these observations really highlight how difficult it is to really behave as if you believed that. So, aspiring rationalists, how might we combat this in ourselves?"

I think there are some hidden assumptions in that question.

Money is supposed to be fungible, yes. But if we act otherwise, is it because money is truly fungible and we're failing to respond appropriately to reality? Or is money itself not fungible and our actions consistent with an understanding that denies our asserted beliefs?

If the unspoken understanding that guides our actions is correct, it's our explicit beliefs that need to be combated. If it's our explicit beliefs that are right, it's our intuition that must be fought. We can't determine the right course of action until we know where the error is.