That line is part of the first page of Og Mandino's book, The Greatest Salesman In The World.
I will persist until I succeed.
That's another line from the same passage.

Both lines address something other than a positive affirmation of the desire to achieve something, but they speak to one of our greatest challenges - overcoming our self-limiting beliefs.

Back in 2013, an online article was published over at Tiny Buddha entitled, "Overcome 8 Common Limiting Beliefs That May Keep You Stuck." 

The author pointed out 8 that were especially vexing to her. They figured out these were the limiting beliefs causing her to be stuck. I suspect it's an all too common list.

* I lack motivation.
* I procrastinate too much.
* I don’t have time.
* I don’t have enough resources.
* It’s too late to change.
* I have too many responsibilities.
* I have no clue who I am.
* I have no clue where to start.

Your list may look different. But you've got a list. We've all got a list.

Dr. Bruce Lipton likes to say that the movie - The Matrix - isn't sci-fi. It's a documentary. :D

So many classic books focus on beliefs - particularly on overcoming our limiting beliefs. I don't know who first put that verbiage to the idea, but we can all relate to it. Nobody is immune.

ChangingMinds.org gives us this insight thanks to a keynote by Alan Stein, Jr.
Limiting beliefs are those which constrain us in some way. Just by believing them, we do not think, do or say the things that they inhibit. And in doing so we impoverish our lives.

We may have beliefs about rights, duties, abilities, permissions and so on. Limiting beliefs are often about our selves and our self-identity. The beliefs may also be about other people and the world in general.

In any case, they sadly limit us.
They go on to say this...
I do/don't
We may define ourselves by what we do or do not do. I may say 'I am an accountant', which means I do not do marketing and should not even think about it, and consequently fail to sell my services well.

Another common limiting belief is around how we judge ourselves. We think 'I don't deserve...' and so do not expect or seek things.

I can't
We often have limited self-images of what we can and cannot do. If I think 'I cannot sing' then I will never try or not go to singing lessons to improve my ability. This is the crux of many 'I can't' statements: we believe our abilities are fixed and that we cannot learn.

I must/mustn't
We are bound by values, norms, laws and other rules that constrain what we must and must not do. However, not all of these are mandatory and some are distinctly limiting. If I think 'I must clean the house every day' then this robs me of time that may be spent in something more productive.

I am/am not
The verb 'to be' is quite a pernicious little thing and as we think 'I am' we also think 'I am not' or 'I cannot'. For example we may think 'I am an artist' and so conclude that we can never be any good at mathematics, or must not soil our hands with manual work.

'I am' thinking assumes we cannot change. Whether I think 'I am intelligent' or 'I am not intelligent', either belief may stop me from seeking to learn. 'I am' also leads to generalization, for example where 'I am stupid' means 'all of me is all of stupid and all of stupid is all of me'. A better framing is to connect the verb to the individual act,