Your subconscious mind is recording everything that you say to yourself so that it can play it back to you.
Sometimes those messages get played over and over and over again, and this "cycling" can help you or destroy you depending on two things.
Imagine that your self-talk is like a tape recorder (yes, I'm old) or an audio recorder app. As you say things to yourself, the two things that will determine if we will succeed in moving forward or remain stuck are the messages we record and the buttons we choose to push.
If the messages (our self-talk) are positive and supportive, they will serve you better and it will be easier to keep moving forward. However, if the messages are negative and critical, you will remain stuck and unable to move on. For example, telling yourself you are learning new things in your new job every day and getting better is going to propel you forward a lot faster than telling yourself that you are incapable and don't know how to do anything because you've never done it before.
The buttons you choose to push can either make those messages a friend or a foe. For example, if you're supportive and compassionate with yourself, and you keep hitting "rewind/play," you will hear that positive and supportive message again and again. However, if you hit "rewind/play," and your self-talk message is critical, all you will do is continue to tell yourself that you are 'not capable' or 'don't know anything.'
Why that's important to understand is because these messages are where your self-beliefs come from.
If you tell yourself you are learning and growing, whether you say it once or twenty times, it can only help you because that's what you begin to believe. But if you keep telling yourself that you are not good at what you do and never get anything right, whether you say it once or twenty times, it's going to make you believe that is true when it is not.
You create your self-beliefs - not anyone else. Once you know that, you also need to know that you can take charge and re-write the message that is being recorded in your own head.
Start to pay attention to what you say to yourself, and then notice if you keep hitting "rewind/play" over and over and over again. If your self-talk is positive and supportive, that repetition will serve you well at times when you need reassurance that you can do something (you actually believe yourself when you say it!). If your self-talk is a bully, hit "stop" instead, and begin to make a new recording!