Reality checker
Prototypes of the basic impulses can be based on simple mechanical processes and "A to B" rules, but an additional ingredient is required to finish their development.
We need a third sequence of data. Expectations. By comparing them with actual feedback from our sensors, we learn if the running program works as intended or not.
It's a simple data comparison. Two sequences in. One out. But the results are important. We can use them to fine-tune our behavior, make it precise and visibly intentional. The most primitive algorithm is to keep the program that works well and replace the one that doesn't work with something else.
This basic reality checker doesn't care about the benefits of your healthy lifestyle. It won't make you eat veggies. It just makes sure that whatever you do somewhat fits the reality.
On this level of evolution, you can already get your first bad habit. You repeat something stupid that works. Also, you can discard long-term plans just because they don't seem to work.
Reality checker requires transfer of information and can be a target for standard attacks on data channels, but all the addictions and procrastinations come with it by design. You may not like them, but technically they're not breakdowns.
What to do next? Replace binary choices for programs with a spectrum of priorities. Add overrides to change them manually from more advanced decision-making units. Define lists of protected programs that can't be overruled. Be creative.
Various task managers can be designed on top of seemingly simple comparisons. They'll have the same core and a species-specific wiring around it, including an optional signaling system to monitor stuff and get more context for decisions.
In the case of humans, there is a so-called reward system that fits the profile. Many other species include its equivalents. If we observe basic impulses in their behavior, they have it.
From the perspective of major diseases, the core checker can be involved both when building a trigger and when suffering from the consequences, but it's not necessarily a cause of all our problems. It's more like a cynical commenter. It merely states the obvious while the world burns on its own.
Malfunction in this unit breaks your ability to synchronize actions with reality. You'll struggle with living. This is a hardware problem on a level way below the one where you can actually construct the central conflicts of depression or schizophrenia.