There are plenty of articles on empathy and compassion, so I don't want to delve into their definitions. I'll use the definitions from this page on the Greater Good Magazine for this post.
Empathy is our ability to take the perspective of and feel another person's emotions. Compassion, which literally means "to suffer together", is when those feelings and thoughts include the desire to help. It is the feeling that arises when you are confronted with another's suffering and feel motivated to relieve that suffering.
These days I'm learning that both require us to have space inside. To welcome someone else's perspective and emotions, we need space within ourselves.
We may run out of space for many reasons.
We may already be filled with our own emotions. Some of them, such as pain, anger, regrets or sadness, can take up a lot of space, if not all of it, inside us. When experiencing those emotions, asking ourselves to be empathic or compassionate may be too much. Maybe we just need to take a walk, meditate or focus on our own feelings until they fade and leave some free space within us.
The amount of space available depends on many other factors, like our physical condition. When we are tired, hungry or ill, the inner space for emotions shrinks until we may not have enough of it for someone's else perspectives or feelings. Again, asking ourselves to be compassionate in these situations may be too much. It's better to take the time to nurture our body, heart and mind to expand our inner space.