Fear is the #1 source of negative energy. In fact, if you trace most other negative emotions back they turn out to be a sub-form of fear. Think about how you react when you are in a scary situation. Psychologists call it a "fight or flight" response. When faced with danger or fear you get an adrenaline rush that helps you do one of two things - either stand and fight or run away. Both are survival instincts, and neither is inherently good or bad. The problem though is that the emotions that come with fighting tend to be anger and fear, and those that come with running are mostly fear. All negative feelings.
This is how demons get their power. As I mentioned, they start of fairly weak in our plane of existence. Just tourists on a bus passing through. They can watch what's going on, but they don't have much effect over anything. Give them a day pass, though, and they'll mix in like they belong.
If you let demons scare you, anger you, or consume your thoughts you are effectively giving them the exact energy they need to get stronger. This is foolish! Luckily energy is not permanent though, and it takes a lot of energy for a demon or other evil entity to exist in our world. Even if you've fed the beast for years with your fear, anger, anxiety, and sorrow... stop. Resolve to rid yourself of the negative energy, no matter how long it takes or how hard it seems. At the very least get to the point where your reactions to scary events and feelings are neutral. Clear your mind and tell yourself that everything is just fine. If you know how to meditate, this is the perfect time to use it. If you don't know how to meditate, learn.
Over time you can actually make yourself unafraid, and with more time you can view spirits with a detached curiosity rather than a terrified reaction. As you learn to accept your own fear of the unknown, you will actually become stronger and will automatically begin to detach from the fear itself.