How can you believe something that you know is true, but are disillusioned about?

For instance, you know that a fact is true... but you believed something contrary for a long long time, but now you have faced mounds of irrefutable evidences supporting the true concept of the fact. However, you find that you are making decisions and forming opinions based off of the previous belief (the false one) and don't seem to find out what went wrong till the decision makes bad results, then you look back and realize how the idea was formed, around bad information.. and realize that although you believe the new correct facts about the idea, the old idea is so ingrained into you that you can't seem to stop acting on instinct in regards to your thought processes, surrounding the old information.