It's good to remember those times when we didn't have that separation and remember that our Dukkha faded away then, so that we do not feel we have to lose anything by the unselfish emotions. It's good to look at what we've learned from our practice and ask ourselves, "What do we really know about happiness and its true cause? What do we really know about Dukkha and its true cause?"
Remembering our investigation into the hindrances develops experiential understanding about Cause and Effect, so when we remember those times we actually weren't under the power of one of the hindrances, when we actually opened with Compassionate/Lovingkindness, and remember that feeling of connection and of well-being. It helps us to remember not to fear it again. It may also be helpful at those times when you feel that separation to open with Compassion for yourself because you are having fear at that time. Opening with Compassion for yourself is very helpful.