[Ennui.]

You’re not a bad person for not having sympathy for them. Just because they may have had a bad home life doesn’t mean that it cancels out everything they did to you. It doesn’t mean that they didn’t make your life hell. And just because they had a bad home life doesn’t give them the excuse to take it out on someone else. Not everyone who has a bad home life goes on to be a bully, and they could have found other ways to cope with what was going on with them.
You can also see where they’re coming from but not forgive them. Sure, they might have had a bad home life for whatever reason. It sounds like your bullying has had a lasting impact on you and as much as people say forgiveness helps people heal, I don’t think it’s required when someone has made you miserable like they did. There’s also no proof that they do regret it or that they remember it at all, but you sure do.
So, I don’t think you need to have sympathy for people who hurt you. You’re not required to validate them.
I’d also tell your therapist that you want to talk about something else in sessions and maybe give examples of things you want to focus on so that you don’t have to focus on bullying. But that’s another discussion!