Hi Ian,
The advice given by Eric, even though it may seem counter-intuitive to you at this stage of your practice, is actually the same advice I would give. Even though the quality of your meditation sessions can be, on the surface at least, viewed as being important, in the overall scheme of things that one is endeavoring to accomplish with meditation, quality comes (is developed) as one becomes more proficient at meditation and at learning the overcoming any obstacles that seem to be put in one's way.
Eric G.:
Quality is very important. The other side of the coin is that, to my way of thinking, the driftiness that you are having in mindfulness needs to be worked thru. Again and again and again.
I meditated twice a day for a while and eventually consolidated into one longer sit. That gives you more time at the cutting edge of your practice.
Every student of meditation goes through this period wherein some sits seem to "do" themselves (are smooth and enjoyable, where the mind is sharp and concentrated and under one's control) and others seem to be "doing" us (are difficult and challenge us to stay focused on the object at hand). This is part of the work one is accomplishing in re-training the mind to follow one's own direction (to do as you wish it to do) rather than allowing the conditioning of the mind to direct how it will go (to do as its conditioning wishes it to do). It is breaking this hold that the mind's conditioning has over us that we are endeavoring to accomplish. Never lose sight of that fact.
The more you bang your head against the wall of the mind's conditioning, the more you begin to
break that conditioning down until the mind
finally, at long last, begins to heed and to follow your wishes as to where it goes and what it maintains concentration on.
Part of this process of reconditioning the mind, for me, came with the practice of
samatha absorption states, which helped to increase overall concentration ability. This led to developing the ability to enter into
samadhi states at will, which has made all the difference in the world. Being able to maintain focus on an object (during calming meditation sessions or
samatha practice) or on a subject (during insight meditation sessions, or
vipassana practice) is what makes the difference in one's ability to finally recognize the truth of the Dhamma.
Every sincere and dedicated meditator goes through these phases in the journey toward self realization. And you are no different.
In peace,
Ian