I was once at a coffee shop where I would often meet someone. I was there alone. For the first time in my life, I felt this principle and understood it. I looked around and no one else seemed to notice. It was all I could focus on.
In the Buddhist principle of impermanence, people will always come and go from our lives. Sometimes they die, sometimes we stop being friends or lovers, sometimes they just move. We will sometimes be in spaces we used to share and now do not.
Allow the absence to be felt, but do so in such a way that can not only appreciate those in your life right here and right now more fully and richly, but also appreciate the gift of sharing time and life and even love with the person who's absence you feel. Appreciation will offer more peace than bitter agony.