How can one keep motivated to stay with the forgiveness process if it is not working after a few months?
First, please keep in mind that it can take many months to forgive, especially if the injustice was severe and you are deeply hurt. I recommend that you focus on your strong will. You probably have had to use that strong will at times in the past, for example, to overcome a soft-tissue injury, or to persevere on a work or school project. Try to remember one incident of appropriating and persevering in this strong will. Now apply it to forgiving. You have a challenge and staying with that challenge by continuing to practice forgiving may lead to even a small improvement in your anger, in your well-being, and possibly even in your relationship with the other person. Any of these as small improvements might increase your motivation of staying with the forgiveness process.
For additional information, see The Four Phases of Forgiveness.