Manifestation is not just a mental thing. It has to do with what we believe about ourselves and the world, fundamentally. We can practice positive thinking or affirmations, but if our life long held beliefs do not match that reality then it will probably not manifest.
Affirmations and positive thinking are tools which can help you retool your beliefs about yourself and the world. Opposite action is another tool useful in interrupting the aspect of feelings attempt to hijack what we want. Our negative beliefs can give rise to negative feelings. We may not feel like doing the thing we want or know to do. That could be your beliefs still trying to sabotage what you want your reality to be.
A lifetime of believing I am the problem, there is something inherently wrong with me, I will never be good enough no matter how hard I try or have much I change does not change overnight. It has taken years of active work to chip away at these deep rooted beliefs about myself, and decades to actually experience my reality differently. but it is possible.