When you repeat an affirmation, with which you are not in agreement, there is often a greater or smaller subconscious resistance, which defeats the affirmation.
In cases of low self-esteem, difficult situations, failure or depression, your mind is often focused on your current situation, and you most probably find it difficult, or almost impossible to think of a different, positive situation.
In such cases, which are quite common, while repeating a positive affirmation you repeat the words, but your mind is aware of your current negative situation.
This makes you more conscious of your problems and the distance between your current life situation and the words you are repeating in your affirmation.
Since the mind is focused on the problem, and not on the positive affirmation, this aggravates the situation.
This does not mean that affirmations do not work; on the contrary, this shows that they do work.
You are focusing your mind on the problem, instead of on the solution, and you keep thinking on the problem or negative situation, repeating it like an affirmation, and therefore attracting more problems or a negative state of mind.
For people who are in a deep negative mental situation, affirmations might not be the right thing.
They need other means to help themselves, such as psychological help or a method to be less negative.
They also need to gain more self-esteem and to strengthen their desire to help themselves.
In case of people who are naturally more positive, or when using affirmations for a goal one is positive about, affirmations would be more useful and will also strengthen the desire and motivation to succeed.
In order to use affirmations you must have no doubts about your ability to achieve what you are affirming.
If it were possible to see what is going on in the mind of people who do things and achieve goals, we would see that they consciously or unconsciously tell themselves they are going to succeed.
They use positive affirmations, often without knowing anything about affirmations.