I talked with a friend yesterday (half-way around the globe for free) and mentioned the Byron Katie inquiry process, and the topic of affirmations came up as well.
Although affirmations obviously can be very helpful for people, it is also an approach of "replacing" less desired thoughts with more desired thoughts. In a way, we are setting ourselves up for a battle among thoughts (as if we don't have enough of that anyway, both within and among ourselves).
The Byron Katie approach instead unravels the believes in thoughts, and allows us to notice and rest in/as pure awareness (a.k.a. the groundless ground, the unborn, the original face, formless choiceless awareness, etc.) And from there, we find a natural and effortless expression in the direction of freedom, engagement, intelligence and kindness.