Mental Health

Typically when this happens, there are a few reasons.

1. the social group is full of trash people. This isn't unlikely, considering they chose your abuser over you. That they no longer want to hang out with you should be counted as a win.
2. They were your partner's friends to begin with. If it's a non-amicable breakup, of course they will choose them over you.
3. You always took a back seat while your partner did all the socializing and meeting people. While you might have felt you had a connection with them, they primarily saw you as "xyz's partner".

Solutions:

1. Stop dating abusers. If you notice abusive behavior in someone, break up with them sooner rather than later.
2. Be more proactive around meeting people and fostering relationships. Go out and meet people and build your social network while you are single. When you are dating someone, continue to go out and spend time doing your own thing with your own group of friends. When you and a partner meet a new group of people, proactively introduce yourself. Actively cultivate and deepen the relationships you have with others.
3. Breakups often lead to one of the ex's not being invited around anymore. While this can be mitigated by (2), sometimes people just feel awkward about spending time around two people who have broken up. If you think this is happening, proactively reach out to the people you like in the group and see if they want to hang out independently - continuing these interactions can lead to you being invited around after everyone's feelings of weirdness have subsided. But sometimes you will just lose a social group, and that's okay, too. If you have been proactive about cultivating relationships, as in (2), you should have other social groups to fall back on.

Then those weren't social groups, they were harassment networks. You're better off learning that now rather than a hypothetical future where you needed their help