Rene Descartes said, "I doubt, I think, therefore I am". Dubito, ergo cogito, ergo sum. You've probably seen the shorter phase, omitting the part about "I doubt", but it's arguably the most important part of the phrase, as it is the impetus for much of our thinking.
We all have doubts. We doubt our ability to do things. We doubt the outcomes of the future. We doubt our very existence. "This could all be a dream". Some of our doubt leads to fear and worrying and unproductive thoughts, that can lead to self prophesying reality. If we're not deliberate about controlling our thoughts, they can and likely will dominate our minds and become our reality.
We can control our thoughts and create our desired reality by countering these doubts with opposite thoughts and declarations that reprogram our unconscious mind. "I'm not smart enough to do that" can be countered by "I'm going to learn what I need to learn to be able to do that". Purposeful and realistic countering of doubtful, negative and fearful thoughts is a must to control our minds and our lives.
The world is constantly programming us in a variety of ways, so be aware that we need to regularly reprogram ourselves to stay positive and focused on creating the futures we desire.