Who you are is determined by your genetics and the conditioning you received from external influences throughout your life. You don't choose your parents. And for the most part you didn't choose the environmental influences that resulted in you becoming the person you are today...
Your voluntary actions can only operate within the constraints of your genetics and conditioning. In other words, you are largely the consequence of nature AND nurture... You have a lot less free will than you think...
Furthermore, you don't choose your thoughts, as you cannot think your thoughts before you think them -- they just spontaneously arise. If you don't choose your thoughts, then you are not the author of them. If you do not author your thoughts, and these thoughts instead are more so mental phenomena influenced by nature and nature, then there is in some sense a lack of moral responsibility for the behaviours that are of consequence of these thoughts...
... and there is an apparent lack of moral responsibility for these behaviours because the individual doesn't have true agency to choose his thoughts, to choose his genetics, and to choose his conditioning -- and if you do not have the freedom to choose, are you really morally responsible for consequences that cannot have been otherwise? Yes, as a matter of cause and effect you are responsible... but morally, ethically? How could you be.
When you realize the harm done unto you by others is merely the result of their own genetics and conditioning, and they couldn't have done otherwise, you open up a path to empathy instead of resentment. The people you meet are doing the best they can with the tools they were both born with and given by their conditioning... For many people, they aren't working with much... and no one has the same exact same toolset as you, so it is an error to judge them and resent them for it.
Now everything thing I wrote is predicated on the claim that free will, in its absolute sense, does not exist. If what I said resonates with you, then your resentment will naturally fade, and be replaced with empathy.
Many people, however, believe free will does in fact exist... and their resentment demonstrates that. They take the harm done unto them personally. How can you not feel angry or hurt and take it personally if you believe the other person could have done otherwise?
Well, it would be hard not to, and this would make forgiveness hard... but what if you chose to forgive? Or are you not free to make that choice due to your genetics, upbringing, and conditioning?
