There is an objective reality out there in the world but none of us have direct contact with it. For any given event each person involved will experience it in his or her unique way. The fact that there are "two sides [or more] to every story" speaks to this fact. Each "side" of a story represents the teller's own version of reality.
So what determines how we experience reality? While there are undoubtedly a number of factors that influence our perceptions chief among these are our beliefs. It shouldn't come as a surprise that our beliefs shape how we perceive and experience things. Here's an example. Imagine two individuals from similar economic and cultural backgrounds. These two people are co-managers for a division in a well performing and well known company. They make the same amount of money. They essentially do the same job. They are both competent and efficient managers. One of these individuals considers himself to be successful and is proud of what he has achieved in life. The other considers himself to be a failure and downplays the significance of his accomplishments. Two people, similar situations, completely different perceptions of reality.
Our basic beliefs -- those that govern how we view ourselves, others, and the world -- are based on our early life experiences. For example, if we're raised in a nurturing home and are given adequate love and affection we learn, "I am a worthwhile person. I am loveable. I deserve to be loved." If we are raised in a household that discourage emotional expression (by, for example, punishing or reprimanding a child for crying or for becoming angry) we learn, "My feelings aren't important" or "Showing emotion is a sign of weakness."
These beliefs then serve as filters. Our brains aren't able to take in all of the information they encounter on a gvien day. They would quickly become overwhelmed if they tried to do so. Instead, our brains use our beliefs to tell us which information is important and which can be ignored or filtered out. Thus, a person with the belief, "I am a failure" would be likely to attach a very high degree of importance to any mistakes he makes but would hardly notice when he does something well.
I'm not one for long, drawn out blog posts so I'll wrap it up. What we beleive determines to a large degree what we experience. These beleifs often act on a subconscious level, just below our awareness. When we find ourselves experiencing chronic negative emotions it might be time to examine our beliefs and determine if they are accurate and/or if they are working for us. If not, it might be time to change them. We choose what we believe. Changing beliefs takes time but it can be done.