Make a list of everything you have ever accomplished. Keep writing until you hit at least 100 accomplishments. They can be as simple as doing the dishes or flossing your teeth every day, or they can be as complex as earning a PHD or raising a child.
Once you have your list, write a positive statement you can say to make yourself believe that you can accomplish anything your desire. Really focus on how you feel. Keep your sentence in the present tense and focused on the self. For example, "I have the power, the wisdom and the support to do anything." or "I love and accept myself exactly as I am."
If you are having a hard time finding a statement that you can believe and feel to be true in this moment, try adding the word 'choose.' The above examples would become: "I choose to know that I have the power, the wisdom and the support to do anything." and "I choose to love and accept myself exactly as I am."
If that is still too big of a jump to believe, focus on spiritual strength and go with the statement: "The power of the Divine (or God) supports me in all ways."
If you have a voice recorder, record yourself reading your list of accomplishments and your positive statement.
Each day, get into a comfortable position sitting or laying down and play your recording. Focus on feeling accomplished as you hear yourself run down your list of how much you have achieved. After the recording finishes, continue to focus on, and feel, your positive statement for a few more minutes. Physically smile while you do this.
If you do not have a voice recorder, do the above, but instead of listening to your recording, read your accomplishment statement.
Repeat step 3 at least once per day, twice would be optimal, for one month.