Affirmations always work. Yes, whatever you positively think or visualize, you will focus on and therefore it will manifest.
For example: "I don't want to be poor."
Assumption: "I am poor."
Result: Thinking about poverty, and being in the identity of a poor person. So the affirmation worked, but not in the way you intended.
Maybe this should be re-phrased in the positive? For example: "I want to be rich."
Assumption: "I am not rich." Who wants what they already have? Want implies not having.
Result: Not being rich.
So we have learned it is better to phrase in the present, not future. Don't use "want" and similar words. Perhaps this makes a better affirmation: "I am rich"? But if you are rich, then this does not work because if you are rich then of course you've already attained being rich. If you aren't rich, it is a lie. Therefore, it doesn't work. There's got to be a better way...
How about: "Suppose I am rich"? Suppose you have everything that being rich means to you. Imagine that this is happening. See yourself in that situation. Then slip into that image of yourself and see and hear and feel what it is to experience being rich.
It works now, doesn't it? It is true, it does not imply or state an undesirable state and it is in the present. At least in your imagination you are rich - you have felt what it is like to be in that wealthy identity and that is a resource you can apply. You've changed your frame of mind and your view of the world has altered for the better. When you have a positive vision that is real to you and genuinely something that you want and identify with, then creative energies naturally flow toward that vision. This is powerful stuff!
Everything in our lives is created newly every time we experience it. Even if it is something we barely notice. Yet we are not aware of making these creative affirmations. There are so many of them that we would be overwhelmed if we had to think of them all. If we wish to break down a wall with a sledge hammer, then we need to create the wall as well as the force to knock it down. We are creating a resistant wall and at the same time creating an opposing force. This may not be the wisest way to do things!
Using this as an analogy, when we are affirming something, even when we do so focusing on ability and being honest, we are making many other affirmations at the same time without being aware of them. We create a problem and try to create a strong enough force to overcome that problem - we can't effectively use force to discover truth but we often try! We make an affirmation but subconsciously we are also creating hidden barriers or "counter-intentions" to that affirmation.
This counter-intention is probably something that we thought in the past was a means to keep us safe, or otherwise make our lives better. It might even be a simple negative thought that we decided at a time of stress and keep thinking unconsciously. Such counter-intentions could be ideas that our parents or our culture bombarded us with as a child and then these became habitual and out of our awareness.
When you are affirming a positive intention it is therefore very helpful to recognize the counter-intentions or hidden barriers that you are creating at the same time. What is connected with your affirmation that you are opposing, or disagreeing with, suppressing or trying to forget?
It is important that you perceive the positive intention (affirmation) as an outflowing creation that meets barriers (hidden counter-intentions), rather than perceiving your affirmation to be resisting or fighting an incoming opposing force.
When these barriers are seen in a clear light, you can then just let them go, or if necessary adjust your affirmation so there is no longer this inner conflict.
It is said that when affirming you should only use positive language and not (for example) "I will not overeat" because the mind will interpret that "positively" and you get the result "I will overeat"; so, why isn't negative self-talk like "I will never succeed" interpreted by the mind as "I will succeed" - why this "double standard"?
The answer is a bit complicated but worth understanding. The right brain, which determines our feelings and hence motivation/action (that the Universe then mirrors through the power of Spirit), interprets our thoughts in terms of the underlying (subconscious) true feelings and therefore doesn't discriminate between conscious acceptance or resistance.
It doesn't listen to a "not" or "never" that's a true observation, but more significantly it interprets based on actual beliefs (at the level of feelings) that are stimulated by the concept of what is being consciously thought - it picks up on what most closely corresponds to the subconscious belief/feeling.
"I will not overeat" is an affirmation based on the feeling that you really do want to eat a lot, so that's how it's interpreted - you really have acceptance of eating even though your conscious mind (left brain) is resisting that. The feeling of wanting to eat is stimulated by the affirmation, so you're more likely to eat more. "I will eat what I want" would be more empowering, as it doesn't conflict with the inner belief and gives you the power of choice.
Whereas, "I will never succeed" may be a deeply held belief, in which case it's accepted subconsciously even though consciously you are resisting that. If you use that as an affirmation it will directly stimulate that inner feeling and as a result you will be less likely to succeed - though if your inner feeling is in fact that "I will succeed", then both "I will succeed" and "I will never succeed" will stimulate that inner feeling of confidence. Say the negative one too much, though, and your inner belief/feeling of confidence of success may start to wane, so of course, it's good advice to stay clear of negative affirmations! But using the affirmation "I will succeed" without first bringing to consciousness and re-evaluating an inner feeling that conflicts with it, will probably not be helpful because it is not really believed.
So the key issue is whether your affirmation is in alignment or conflicting with your deeply-held belief. That's the real reason why affirmations don't necessarily help but may indeed increase inner conflict and serve to suppress the inner belief rather than resolving it. When you have uncovered the inner belief and genuinely discovered it is false - not rational/helpful/true/your own - then using an affirmation can help to keep in the place the revised belief, and that's where affirmations are indeed very helpful.