Life is coming at us all the time, from all directions. We have to choose which problems we let get to us, which situations we tackle head-on, because otherwise we would probably spontaneously combust. As fun as that sounds, I think I, for one, would rather keep my body in one piece!
One piece of advice I've been given recently during my job search is to learn not to let everything get to me. People who deal with the public often have to handle a million conflicts in even a single shift at work, just because any place where people gather, problems will also accumulate. This means that someone who became a waiter, for example, would have to know going in that they shouldn't take everything their customers say to heart. A person in this situation has to decide what specific circumstances matter enough to get worked up over and what's insignificant enough to let go.
I've had to face helpless situations before, and it wasn't even in "dealing with the public". I've had a friend make a habit of coming to me for advice about circumstances where they already knew what to do (and had usually already decided to ignore it). I've had someone cry on my shoulder and cling to me for comfort, but I couldn't talk to them about the situation when I knew that anything I said to them would be a waste of our time because they weren't able to take the advice of others (and will have to decide someday to change on their own). I've even had someone come to me with their problems and try to tell me that they were all my problems too; they actually said that I had caused most of them. In all of these situations, I would have loved to have been able to help, but it was impossible. I had to consciously decide to take an emotional step back. I tried obsessing over the issues before I learned better, and all it did was make me miserable, which still didn't help the other person.
Sometimes, a friend may come to you with a problem, but there's simply nothing you can do to solve it or help them make a decision. In those instances, you have to distance yourself by realizing that it's a waste of time to stress yourself out over something you can't change. Whether you're dealing with a random customer or your best friend, you can't fix everything.