If our heart shuts down when thinking of one sentient being and we can't bring ourselves to include them in "all," we should stop and observe what's happening in our heart/mind. With compassion for ourselves, we ask, "What in me is resistant to this? Am I hurt? Angry?" When we become aware of what we're feeling, then we apply the Dharma antidotes, for example remembering that this person hasn't always been who they are now. In our past life, he was our best friend or someone who rescued us from danger. Thinking like this, we begin to see him as kind in that way, and our heart can open and wish him well.
It's important to remember that "all sentient beings" includes us as well. We are part of "all." Sometimes we forget to include ourselves in the scope of our equanimity, love, compassion, and joy, and in the end that leaves us feeling dry and isolated. While we want to overcome the wish for only our own happiness, let's not go to the extreme of wishing for everyone's happiness except our own.— Thubten Chodron, Cultivating a Compassionate Heart