Many times we make the mistake of thinking that we can overcome mental obstacle with sheer willpower, but our brains are actually programmed against us. Our brains are ancient (or so scientists keep telling us) and so are programmed to act as though we are still ancient creatures living in primitive conditions.
For example, ancient humans, to survive, couldn't afford to lose. Our brains remember that, even though we are modern creatures. For example, have you ever ordered something in a restaurant and then feel like you have to eat the whole thing, even if you're not enjoying it are full? Of course you have. That's called the "sunk cost fallacy" and makes us prioritize what we've lost over what we could possibly gain.
There's also the "optimum bias" phenomena, as well as "confirmation bias. Here's the full article, including a short, three-minute video that explains it better than I could.