It can be easy in meditation practice to rely on what are called avoidance intentions. These are intentions framed in terms of what you want to avoid. For instance, you may find yourself trying to prevent the experience of metta from fading, or preventing distractions from arising.
This is natural; we don’t want to be practising meditation ‘wrong’.
However, avoidance-oriented goals tend to manifest in scanning for mistakes and imperfections to correct for. They force you to look for what you want to avoid. One becomes quite paranoid and tightly strung practising this way; there may be some doubt that something has slipped under the radar. And what you look for, you’ll tend to find! You can virtually always find at least a little something wrong with your practice, and so always feel like you’re falling short.
A helpful alternative is to practise with “approach-oriented” intentions.