Usually failure is a lot less terrible in reality than what you imagine.. or when it is terrible you tend to very prepared for your task. If you have too much optimism you don't tend to be prepared enough and when you fail it is hard to try again. Being realistic and basing your life on reality is much more powerful than being an optimistic wishful thinker.
Deep down your true self knows you are trying to deceive it.
And remember what you can really accomplish if you just progress one little step at a time..
if you imagine a failure you can surely imagine a way to avoid the failure.. that is really the point.. to be prepared.. and also to be prepared to realize that the failure is not really not so bad and is really a part of every endeavor.. without it there is no success. If you imagine you always only succeed you will accomplish a lot less.