I am absolutely no expert on mindfulness or meditation. I've never been on retreat and I might have spent about as much time reading (and listening to podcasts) about the topic than actually meditating. (Well, that's probably not true.)
This is probably just my bias, but I think that the intellectual understanding of what mindfulness is going for is more important than actually meditating (after a point). That is, I think that using meditation all day long is the goal, not sitting on the cushion for some time (or the bench, one of which was hand-made for me).
Not that I'm anywhere near enlightenment, but I think that to get the "point" of mindfulness meditation, you have to embrace a paradox. The first step is realizing that "thoughts think themselves." We don't have any free will, no homunculus in there making choices of what to think (or do). The "self" is, ultimately, an illusion.
My thoughts come and go. I do not think my thoughts. My thoughts are not me; I am not my thoughts.
And yet we can "choose" [sic] to program ourselves such that, at some level, we can observe our thoughts. Like someone asking, "What are you thinking about?" except that we do it ourselves.
Observing our thoughts and realizing the thoughts are not us, we can "choose" to let negative thoughts go. We don't have to hold on to any thought (except when a Hamilton song* gets stuck in our heads), so can "choose" to drop negative thoughts and replace them with recognition of beauty and good fortune.
Of course, this is easier said than done. I have noted that depression is an issue that can, at least sometimes, be treated with drugs. (The side effects of my current antidepressant have gotten to the point where I'm probably going to dry a different one.) I have been able to recognize depression and negative thoughts, but not be able to mindfulness my way out of it. (Kinda like trying to stay awake for days based on "willpower.") It is only after getting to a better place with the help of medication that I'm able to use mindfulness to any good effect.
I could clearly get much better, but that is my direction.
*Not a bad song to have stuck in your head: