CBT can treat severe depression, but the problem is that all those deep-seated underlying mental patterns don't just change because you want them too.

Most people don't seem to realise that the celebrated prefrontal cortex is the youngest and in a way weakest part of the brain. It surrounds older structures that don't work much different than in reptiles.

Changing your thoughts is easy, changing the attached emotions, fight/flight responses etc., who might even trigger the thoughts in the first place is incredibly hard.

It takes time, the right corrective and endless patience and iterative practise.