Change karma?

Many spiritual teachers say we can change karma, a friend suggests me to find a shaman to clean my aura and change my karma, too.
I also always believed the free will can change our karma.
So i tried very hard on my will, and my life did get better, but after years and years i feel the wheel of my life still on this designed map--karma, I now start to realize why many eastern people say Karma=destiny.

My father was a horrible man, he abused mother in different ways and made her physically mentally ill, but now my father changed to another person in his old years, something just happened to him , a big surprise ....he becomes like a baby character , naive and peaceful. my mother however becomes easy to angry and always in bitterness.
The family members say my father must have done good things in past lives he has a good karma. ,my mother however not a good karma.
but my mother always tries her best , and she is loving by nature and always tries to help the poor.

So, I m wondering if karma is destiny, we need to obey it and then we will be better in after life?
If karma isn't destiny and we can change it, how ? and how much percent can be changed?

Thanks a lot!
Have a good day on our journey.