The purpose of insight meditation is to reduce the defilements of the mind, namely, greed, anger and delusion. If these qualities of the mind are reducing, then we know that our practice is working successfully.
Often slow progress is the best progress. The reason for this preference is that when we are trying to achieve positive change we want those changes to last. We want to be an enduring star rather than a shooting star.
To overcome doubt and remain confident, we need to see progress. When progress is fast, the changes that we experience are so obvious that we do not need to put in much effort in order to discern them. However, when progress is slow, we need a more systematic way of determining how we are doing.
Mindfulness, as usual, provides us with the solution. To see how we have changed over time, we simply need to observe our current situation, our behaviour and how people are reacting to it. Then, comparing those to our memories, we can quickly observe whether or not there has been a positive change.
If there has been little to no change, it is probably because we have not been devoting resources to meditation practice. The principal way to make meditation a priority is to train in the technique at a regular meditation retreat. If we do that, we can then use our skills in mindfulness to confirm its inevitable positive effects.