Speaking as a phenomenologist, it seems to me that a considerable portion of mind wandering actually is "mental avoidance behaviour", an attempt to cope with adverse internal stimuli or to protect oneself from a deeper processing of information that threatens self-esteem.

Imagine you are trying to lose weight and attempting to concentrate on writing an article, but there is a bowl with your favorite chocolate cookies in your field of vision, a permanent immoral offer. If we are capable of rejecting such offers or to postpone them into the future, then we can also concentrate on that which we currently want to do.

The self is not a thing, but a process.

The notion of a conscious model of oneself as an individual entity actively trying to establish epistemic relations to the world and to oneself, I think, comes very close to what we traditionally mean by notions like "subjectivity".

What many people don't see is that there are abundant examples of phenomenal opacity: It is one of the most interesting features of the human conscious model of reality that, first, it can contain elements that are not experienced as mind-independent, as unequivocally real, as immediately given, and second, that there is a "gradient of realness" in which one and the same content can be experienced transparently or in an opaque fashion.

As a first-order approximation, I would say that phenomenality is "availability for introspective attention": Consciousness is a property of all those mental contents to which you can in principle direct your attention.

The Ego is a transparent mental image: You, the physical person as a whole, look right through it. You do not see it. But you see with it.