I think that humans can mentally self-modify to some extant, especially if it really really matters. If you really needed to be optimistic, you might be able to modify yourself to be such by significantly participating in certain types of organized religion. (This is a rather extreme example -- a couple minutes of brainstorming would probably yield ideas with (much?) lower cost and similar results, but it illustrates the possibility.)

Expected utility maximizers are not necessarily served by updating their map to accurately reflect the territory -- there are cases such as the above when one might make an effort to willingly make one's map reflect the territory less accurately. The reason why expected utility maximizers often do try to update their map to accurately reflect the territory is that it usually yields greater utility in comparison to alternative strategies -- having an accurate map is (I would guess) not much of a source of terminal utility for most.

ETA: Missing words. >.<