Compassion fatigue or empathy fatigue?
It's ironic that people use the term compassion fatigue to describe the depression, sleeplessness and anxiety that may come from being immersed in other people's suffering. Compassion is the antidote to the burnout and stress caused by overuse of empathy.

The Folly of Empathy
Empathy is to "relate" to the events that caused else's suffering - to try them on and think about how I would respond if they were happening to me. The first disaster is that I am now embodying the torture. Brain studies show that you can practice something in your mind - say, throwing a football - and it will actually result in a change in how you perform in the next game. While this might be great for athletes, it does neither of us any good if I imaginatively join your story of being raped or beaten. The second disaster of empathy is that it sets us judging and assessing. "Would I be as distressed as them if my pet frog died?" "How can this person be joking; their kid is in the hospital?! If I was joking at a time like this, it would mean I..." Empathy brings the focus to myself. Not just to the self, but to the self as distinct - as similar or different - from the person before us. This is the very definition of ego. This is not to say that empathy is useless. Empathy is important when you are considering a course of action. Empathy suggests we look at the world and ask how it would be if what we plan were to happen to us. The proper use of empathy is to still one's hand. Empathy is a tool for planning, not for helping.
The Spaciousness of Compassion
If we are bearing witness, empathy might be better replaced with compassion. Compassion does not require that we determine the worthiness of the other person's distress. Compassion meets distress as it is, in the present moment, in the human before us. It ignores the absolute value of the event, and keys on the presenting human's ratio of pain to capacity. If I'm being compassionate, I respond in similar ways to someone who is 90% devastated, whether I am dealing with a child who lost a replaceable toy, or a parent who lost an irreplaceable child. Compassion focuses on the person who is suffering, in the way they are suffering, in the moment of their suffering. I don't need to think of how I would handle the instigating event, only to know that yes, I have been 90% upset. The less I compare myself to the person in front of me, the less opportunity for ego fatigue. People suffering from "compassion fatigue" are actually suffering from "empathy fatigue."
The Shield of Humility
If I am empathizing, I imaginatively put myself into your struggle. I want what is happening to stop as if it were happening to me. The more I do this, the more desperate I may become; I lose my capacity for acceptance as I fumble around for a plan to get "us" out of here. Boundaries blur. It becomes hard to dignify the struggle of the person in front of me. Instead, I want relief that may not come. This sets me up for compassion fatigue. One sign that this is happening is that I begin to work harder for solution than the person in front of me. The person I am "helping" goes back in, back to the drug or the relationship, or maybe their situation has no relief available - perhaps they are dying, or their suffering will go on unabated until it is ready to end. If I am in empathy this can be almost unbearable. Truthfully it is often more painful to witness or imagine things than to experience them for yourself. No wonder good, but unskillful, helpers can collapse. The antidote is humility. Not knowing what "needs" to happen releases me from the responsibility to play god. Looking back at our own lives, most of us can see how necessary it was that we went through some of our worst moments. When I listen from compassion, I can rest in the present moment. Without the requirement to fix events, I can provide space for you to hear yourself speak, or feel your feelings (or avoid them). In this space I can wonder out loud where this needs to go - and hold the silence open while you feel your way into your own answers. Compassion is not there to pull you out of the pool, compassion swims along side with a flashlight. I believe that eventually, all people who are extremely compassionate learn these skills - but it can take a lot of time and suffering to get there.