Look forward to win it after you win it.
You are going to win it when you won it.Interesting!
So the past comes after the future?? Didn't get it.

well, look...you should look into it little deeper, hon.
It wants to say that nothing is done till it's done. So, do not start enjoying something till you get it. Not in advance; regardless how much it is apparent to happen.
So, it is an advice how not to start enjoying/contemplating something before you got it. If you do, then you run the two huge risks:
1. Disappointment: in case if it didn't come true.
2. Loss (mental or material) because of wrong planning/expectation.
These two risks are TOO huge for anyone to "afford" them. That's why the saying because it teaches us how to avoid the losses duw to great expectation. So:
no great expectation = happy and successful life.
hope this helps.