In tonglen practice, when we see or feel suffering, we  breathe in with the notion of completely feeling it, accepting it, and owning it. Then we breathe out, radiating compassion, lovingkindness, freshness - anything that encourages relaxation and openness.  So you're training in softening, rather than tightening, your heart. In this practice, it's not uncommon to find yourself blocked, because you come face to face with your own fear, resistance, or whatever your personal "stuckness" happens to be at that moment.