When we lament about things done/left undone, of course there is always the temptation to make it about ourselves (it’s the unfortunate universal human gravity to do that). But if we are aware of the temptation, more often than not we can avoid it. And avoid it we must; otherwise we’ve got no Good News for ourselves or to share with others if we make it about ourselves.
So when I reflect in the manner you often hear others do, it is not always about self-loathing or trying to achieve some kind of works-(self) righteous status with the Lord (an impossible thing to do as you point out). We indeed are made alive by the blood of the Lamb shed for us. It’s the only reason we are alive and can have any real hope.
But we are also called to come and die as we follow Jesus and that means ruthlessly rooting out all that is in us, with the help of the Spirit, that keeps us from wholeheartedly following Jesus. Jesus himself tells us we must do this in the gospels and in the first three chapters of Revelation he reminds his Church in no uncertain terms that this is what we must do if we are to be called his followers. This requires being honest with ourselves and often requires confronting some hard things in ourselves so that we can put them to death as our Lord commands.
We don’t do this alone, of course, but do it we must. Based on a careful reading of the whole of Scripture, our challenge is to keep a healthy balance of both sides of the same coin. God has indeed won the day for us in Jesus. But how we respond to his gracious love poured out for us is important to him too. We can’t have one without the other; otherwise we lose the faith that was once for all entrusted to the saints.