How will we come to terms with killing people? or a past family member now infected? no matter how much we talk about it, it wont mean anything when the actual experience comes around. I am just starting a thread, I don't know how we can train ourselves to not care.

Simply working off of what GargoylesLover said.
Letting grief take over is the one most dangerous thing you can do. With time hopelessness begins to set in, self doubt begins to become dominant. Being killed by zombies, people, or natural causes becomes unimportant to you as time draws on due to the constant belief that there is no hope.
Things like suicide and insanity result from letting grief, anger, or any other emotion heavily take over.

There is no way to "train" your mind to deal with the stress of undead combat.
It is something that must be done during the crisis, and it is the single most thing that kills the most people.

People well supplied, well armed, but weak-willed.