I know exactly what I should be doing and I still do not do it. Why?

Because knowing and doing run on different machinery, and the gap between them has at least four distinct causes that feel identical from the inside. Which one you have decides what works, so the useful move is narrowing it down rather than trying harder.

The first is initiation. Some people are organised and structured but poor at starting — planning and follow-through are separable, and they correlate much less than the word "discipline" implies. If this is you, more planning actively makes it worse, because planning is the thing you are good at and it becomes a sophisticated way of not starting. What moves it is making the first action small enough that it does not require a decision.

The second is avoidance of a specific feeling. If the tasks you skip are consistently the ones where you could be judged or could fail visibly, the behaviour is not a discipline problem — you are avoiding an emotion, competently, and the plan is irrelevant to it. The tell is that the avoided tasks cluster by emotional content rather than by size.

The third is that the goal is not actually yours. Intentions inherited from what you think you should want produce exactly this pattern: real agreement at the level of the sentence, no pull at the level of the action. Worth testing by asking what you would lose if you never did it, and noticing whether the answer is a real cost or an abstract judgement.

The fourth is straightforwardly load. Follow-through degrades hard under poor sleep, stress, and too many simultaneous demands, and people consistently read that as a character failure. If this started at an identifiable point, look at what else changed.

One thing worth ruling out before any of it: what your completion rate actually is, rather than what you remember. People who describe themselves the way you just did are frequently completing more than they think, and the gap is between behaviour and self-image rather than between intention and behaviour. Those need opposite responses — and the second one does not improve by trying harder.