You believe it is possible to achieve. You take positive action to get there. Affirmations support you in the belief and in the positive action. This is especially useful for people who tend to be pessimistic or self-doubting. Affirmations are valuable for developing the habit of focusing on what you have and what you want to have and what you want to do.
Affirmations work best when stated in the present tense.
You have been told that it is vital to your health to eat a healthy diet after the surgery and you are afraid you won't be able to stick to the diet. If you keep worrying and repeating your doubts to yourself, you run the risk of programming your unconscious mind to expect failure and to work towards failure. Repeating the right affirmation can turn your expectation from failure to success. This supports you in staying on the diet. It supports you in expecting to succeed.