What you internalise will manifest itself. If you think you will not make it, you most likely will not. And if you think you would, then you might very well find yourself pleasantly surprised. Of course, this is done within reason. If there is no chance left due to limitations that cannot be regained back like time, then no amount of optimism will move the outcome to your side. But if there is, no matter how slim that may be, and you do not discount the possibility of you succeeding despite that small odds, you might very well just succeed. When your mind believes that there is still hope, it will do whatever it can to get you to succeed. So do not throw in the towel before giving it everything you got. Believe that you can, and you just never know.