First of all, karma is not an excuse. It should not be used as an excuse. My method is to never believe in bad karma. Always believe in good karma. Bad karma does not exist. Only good karma exists. If you integrate enough and push hard enough with your wisdom, practice and dedication, all the negative karmic habits will dissolve naturally. That is the reality and very nature of the Buddha himself. If we keep on being depressed with this bad karma, I am sure there will be no progress with good karma at all. That is why the Buddha continued with eons and eons of believing in gratefulness, in positive and harmonious ways, in bodhicitta. He was not depressed all the time.
From my first book < I Am a Lost and Found Buddhist >
Love and respect from your Kalu Rinpoche