knowing your future beforehand would change the future from what you would have experienced otherwise: that one difference, that change in knowledge, would result in another change leading to another and another, and all that would accumulate over time,
completely altering your fate.
..not to mention the potential paradoxes. for example: could you change the future from being the future? because if you do, then it wouldn't be the future anymore, and consequently not the future you were told in the first place..