The irrational aspect is not that there is conflict between the short-term and long-term or that you act based on short-term consequences, but that when you think rationally while contemplating eating the chocolate bar about the short-term, medium-term, and long-term consequences, weighting them all appropriately, you still end up eating the chocolate bar even though at this very moment your careful consideration of the weighted factors led to the conclusion that not eating it is preferable.