While the idea that maintaining an "I can" attitude may seem like a stereotypical self-help mantra, there are actually substantial physics-based concepts that support this rather simple idea. Manifesting is all about energy. The more energy the better. An "I can" attitude increases your energy level while an "I can't" attitude decreases it.
In this way, the momentum of your trajectory towards manifesting your desires becomes directly defined by the manner in which your thoughts influence your personal energy value. Positive statements that increase your energy add to your momentum towards fulfilling your desires thereby directly increasing the rate at which your desires are being manifested by your thoughts while negative thoughts that detract from your energy value decrease the rate of your momentum towards their fulfillment.
Therefore, believing that you can manifest your desires becomes a requirement under certain circumstances since the thought of anything other than the belief that you can accomplish your goal only hinders your ability to do so. Thus, the more adamantly do you believe you can accomplish a certain goal, and the more persistent is your focus in the accomplishing of it, the greater is your momentum towards its fulfillment due to the additional energy such positive thoughts provide.
A belief in your ability to accomplish a certain goal also acts as a gauge as to the degree to which you are being positive in general too since a more adamant belief in your ability to accomplish your goal always coincides with a greater degree of positivity in general. Due to this constant correlation between positivity and your belief in your ability to achieve a particular goal, either one may be altered to increase the other.
For instance, you can increase the degree of your belief in your ability to accomplish a particular goal to increase the degree of your positivity in general, or you can increase the degree of your positivity in general to increase the degree to which you will believe in your ability to accomplish your goals. Thus, the more adamantly do you attempt to convince yourself that you truly can accomplish your goals, and the greater is the degree to which you are able to increase the positivity, desirability, of your thoughts in general, the greater will be your belief in your ability to fulfill even your grandest of desires.