It's possible that naming the part of us that makes us lose is oversimplifying the problem. We can consciously come up with rationalizations for why achieving one result counts as a "win" and another result as a "loss". But clean win/lose states don't exist in real life, which is much more messy. Instead winning and losing is achieving different results.

Is it possible that Bruce is just playing a different game, rather than solely attempting to make "me" lose my game? Bruce may actually be the person who wants things that we can't easily rationalize that we (me and Bruce as one person) want.

I can play a game of bowling against a beautiful woman and tell myself "I want to win this game". However, if Bruce has reason to think that losing is going to help my cause with the woman more than winning, and Bruce knows he wants that woman, then he may try to win his game at the expense of me winning mine. My lazy brain can't come up with the reasons why Bruce want's to throw a gutter ball (to get the woman) and Bruce can't figure out why I'm trying to throw strikes (to achieve my conscious win-state). If Bruce wins then my conscious mind is mad at Bruce for causing the loss without understanding why he did it (my brain is too lazy to figure it out).

Maybe if I beat Bruce then he may similarly be able to make my victory bittersweet without me being able to rationalize the reason for it, thus giving more reason to cave to Bruce in the future.