Empathy is simply feeling others' emotions as your own, and many people feel it to some degree. Several people will feel it to a high enough degree at least once for it to effect them negatively. By this I mean, headaches, heartaches, random instances of anger, anxiety attacks, and various other poor reactions.
Learning to control the empathy in such a way that it will not randomly effect you, or will not do so negatively is something both important and necessary for those who have problems with their empathy. If you are working with your empathy often in order to increase your empathy towards a single person or towards many, I highly recommend you learn to do this as well.
Honestly, controlling your empathy isn't overly difficult. The easiest way is to use a particular visualization to both allow and disallow the emotions of others to effect you.
Simple ways to do so are using shields that are intended to block out only the negative or all emotions if you can't handle it. With shields, if you want to open up to someone in particular the easiest way is to program it so that it will allow certain people in when you want to feel them or those certain people in general.
You can also use things such as a door or doorway, a light switch, or (in my husband's example) a hallway full of doors that you can choose to open or close. It doesn't matter what kind of visual you use, only that you put the energy and intention into it to block out your empathy to a certain degree.
If you have worked with your links to people and only feel the empathy towards them, you can use a filter visual to block then out slightly or a closed door to block them out completely. Please note, you cannot do this in all instances at all times. There are certain links that are difficult to work with.