Why people wont forgive can be for a number of reasons @Bunniekins .
They may not prepared to forgive ... yet. This can be due to depth of wounds or they want their forgiveness to be genuine.
Sometimes by not forgiving they maintain control of a relationship, playing the victim gives them a false power as they have the moral high ground.
I might be wrong but it sounds as though what you have been experiencing is more than not forgiving but also revenge (have put in link to an article that explains why people are vengeful). Not being able to forgive is different to bad mouthing others and spreading unkindness.
Others may know how to handle this better than I do - if you are seeing a therapist they may be able to help.
https://www.psychologytoday.com/au/blog/tech-support/201707/the-psychology-revenge-and-vengeful-peop...