IF you think you can, you can.. or not?

When I was a little kid, someone told me this "if you think you can, you can!!!(with the exclamation mark)". This phrase is good if you want to motivate someone, building the confidence in someone. So I always use this phrase to motivate myself..

When i was in primary school, i hate arts, in fact till now i still hate it..
Aih, how to draw? Don't know how to draw ler..
If you think you can, you can!!!

In college..
die lar, nv study, dono will fail or not..
If you think you can, you can!!!

At work..
need 7 days to finish this work but ppl only gives you 1 day to complete?
If you think you can, you can??? (with the question marks)

I don't think this phrase works for me anymore..