Your description of what happens when it doesn't go right sounds very familiar. For years during retreats I struggled with sleepiness, I never knew when it was going to pop up. Usually after several days it would creep in, I tried numerous things with no success. Then I had a breakthrough. The day before I left for a 10 retreat I started to come down with a head cold, one day in I knew it was going to be a strong one. I was bummed, nothing like spending a retreat sniffling, sneezing and blowing your nose. But it did not go as I expected, in fact that retreat was a turning point in my practice, I learned something very valuable. During the sits my nose would run, not enough so that I would have to blow it but it was always wet. My wet nose became a strong meditation object to watch and watch it I did. All kinds of strong sensations became connected to the breath, my concentration deepened and my nemesis, sleepiness, never became a problem and hasn't been a problem since. I learned that meditation requires a combination of concentration and precision. Concentration came easy to me, I typically had little problem following the breath, it was the precision that I lacked. With my wet nose I was able to look closely at the sensations of the breath flowing through my nose, I learned to examine them in finer and finer detail, going into them deeper and deeper. Now, when I feel a little sleepiness starting to creep in I know that I am not looking closely enough at my meditation object, I haven't lost it, my attention is still on it, but I am not really in close contact with it. Your description of your practice seems rather generic, watching the breath come in and go out. I suggest that you try to tighten it up a bit, look closer at the sensations that you label as 'the breath', examine them in finer and finer detail, become interested in them. That's what worked for me.