Forgiveness is a precious thing that cannot be given to everybody.
Here is my method. It is a line of reasoning.
I start with..
Premise 1. everyone wants to be happy or have contentment within their life.
Premise 2. everyone comes up with methods of obtaining happiness or contentment.
Premise 3. the method a person selects to obtain their happiness or contentment can cause you or others problems as a result.
Premise 4. If everyone was by default already content and happy in their life they wouldn't do things to cause you problems.
Premise 5. Some people purposely select methods they know will cause problems for others because the desire for contentment or happiness is over powering or valued more than the harm they are causing.
Premise 6. Some people unknowingly cause problems because they do not factor in the potential to cause others problems in their pursuit of their happiness or contentment.
Premise 7. Realize that you and everyone else are striving for contentment or happiness as goals to achieve but no one knows the best method that does not cause others problems, if we did then no one would ever cause problems for others unless their contentment or happiness is based on causing problems as the goal.
Conclusion: When you see that everyone is in the same trap that we live our lives in seeking happiness and contentment for ourselves that we will occasionally cause problems for others both willingly and unknowingly. This gives you a basis for forgiveness because you can see that even though they are causing you problems they just want to be content or happy and they not necessarily would have caused you these problems if they were happy or content to begin with.
It is not a perfect line of reasoning but it helps you to put yourself in other's shoes to understand their motivation for their actions no matter how cruel or wicked they might seem. You will see that they were seeking happiness or contentment but went about it with a bad method to obtain it. Feel sympathy for them and empathy for our shared curse that we all just want to be content or happy with our lives.