In fact, it is seldom the case that the past was "objectively" better. Psychologists refer to this misconception as rosy retrospection, which is a well-studied cognitive bias. It appears because, when reflecting on the past, we are more likely to think about people, events, places and things in the abstract. This means we are more likely to focus on positive generality than harsh and sometimes cruel details.
One example: If you reminisce about a family vacation five years ago, say, you will probably recall the pleasant conversations you had, good food and great family photos. You are less likely to remember sleeping in an uncomfortable bed, or bad weather on a trip to a nearby island.
Negative details about past events gradually leave our memory, while positive aspects of past experiences remain. This is good because it keeps our current mood positive. If the mind did not fulfil this function, we would be much less likely to go on the same vacation next year.
In fact, people who remember negative experiences more are likely to suffer from mood disorders, such as depression. In other words, rosy retrospection is a cognitive distortion that serves an important purpose.
On the other hand, cognitive bias can lead to failures of proper decision making. This is one of the reasons why, for example, we repeatedly enter into problematic relationships. The longer it has been since we experienced the negative effects of a previous bad relationship, the more likely we are to let good memories outweigh bad ones and perhaps forgive unforgivable behaviour.
It is always a good idea to approach nostalgic feelings with a healthy degree of scepticism. Many years of psychological research generally suggest that our best days are ahead of us, not in the past. For instance, one study looks at the trajectory of people's optimism over time. Researchers found that optimism was lowest among humans in the 1920s, then rose gradually in the 1930s and 1940s, peaked in the 1950s, and then gradually declined. Another study found that life satisfaction in Anglo-Saxon countries tended to improve with age.
In other words, there is good evidence that your happiest days are yet to come, even though you may feel miserable right now and things are not going how you want. Don't avoid thoughts of the past but do take your memories and feelings with a grain of salt. Don't use the past as an excuse to be unhappy in the present.
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