When the principle of making acceptance of one's own unique weaknesses the essence of the self conflicts with the principle that enjoying joyful achievement in a well-designed environment contributes positively to mental health, which principle should take priority?

I'm not quite sure I see when the two statements would conflict, given that they appear in the same context and in the same vein. As an example, I can imagine someone who, with the attitude of 'I'd rather tackle easy problems where I can shine than agonize over understanding or embracing my weaknesses,' throws themselves into games or particular real-world problems. In that case, I think the former should obviously take priority. Being told to restore one's mental health is advice that applies temporarily, when one's health has collapsed; it is not a suggestion that once one has recovered, one may keep indulging only in short-term pleasures.