So if our lives are determined by our karma, can we change it?
 
Of course we can. That is why one of the steps on the Eightfold Path is Right Effort. It depends on our sincerity, how much energy we exert and how strong the habit is. But it is true that some people singly go through life under the influence of their past habits, without making an effort to change them and falling victim to these unpleasant results. Such people will continue to suffer unless they change their negative habits. The longer the negative habits remain, the more difficult they are to change. The Buddhist understands this and takes advantage of each and every opportunity to break mental habits that have unpleasant results and to develop mental habits that have pleasant and happy results. Meditation is one of the techniques used to modify the habit patterns of the mind as does speaking or refraining to speak in certain ways, and acting or refraining to act in certain ways. The whole of the Buddhist life is a training to purify and free the mind. For example, if being patient and kind was a pronounced part of your character in your last life, such tendencies will re-emerge in the present life. If they are strengthened and developed in the present life, they will re-emerge even stronger and more pronounced in the future life. This is based upon the simple and observable fact that long established habits tend to be difficult to break.
Now, when you are patient and kind, it tends to happen that you are not so easily ruffled by others, you don’t hold grudges, people like you and thus your experiences tends to be happier.

Now, let us take another example. Let us say that you come into life with a tendency to be patient and kind due to your mental habits in the past life. But in the present life, you neglect to strengthen and develop such tendencies. They would gradually weaken and die out and perhaps be completely absent in the future life. Patience and kindness being weak in this case, there is a possibility that in either this life or in the next life, a short temper, anger and cruelty could grow and develop, bringing with them all the unpleasant experiences that such attitudes create. We will take one last example. Let us say that due to your mental habits in the last life, you came into the present life with the tendency to be short-tempered and angry, and you realise that such habits only cause you unpleasantness and so you make an effort to change them. You replace them with positive emotions. If you are able to eliminate them completely, which is possible if you make an effort, you become free from the unpleasantness caused by being short tempered and angry. If you are only able to weaken such tendencies, they would re-emerge in the next life where with a bit more effort, they could be eliminated completely and you could be free from their unpleasant effects.