The Doctrine of Karma or Kamma is not a mystical force and does not entail fatalism. Instead, it is a natural phenomenon, like gravity. Our thoughts create consequences inside our mind which we then act on. The doctrine refers to our intentional mental actions- our volitions. What we are now is determined by our thoughts and actions in the past and what we do next, in the future, is determined by our thoughts and actions in the present. Therefore, our kamma has the potential to continuously change depending on the development of our thoughts and actions.
The Buddha was very clear in teaching the Noble Eightfold Path that we can transform the quality of our mind and action for the better and ultimately achieve Enlightenment. So, Karma does not mean that we have a fixed destiny across lifetimes that we must passively accept or that bad or good things happen only because of our past actions.