First and foremost, it's good that you're no longer dating someone who's jealous, controlling, and possessive. All those three are forms of emotional abuse and can scar someone for a long time, especially without therapy, counselling, or intervention.
However, as glad as you are for being out of the relationship, him being with a new girlfriend can worry you. Knowing how he acts, you wouldn't want anyone to experience the same thing. Unfortunately, you can't change someone who is emotionally and/or physically abusive towards you.
If you are afraid he's treating his new girlfriend the same way he treated you, there are several tips on how you can help her:
If you're not actually sure of the relationship being unhealthy or that she's not showing signs of being abused, you can warn her subtly. Tell her a little about how he treated you, but not so much that you'll scare her off. (Also read 19 Characteristic of A Healthy and An Unhealthy Relationship.)
This mainly goes for your ex boyfriend. You don't want him coming back to your life or suddenly have him show up on your doorstep demanding why you're intervening in his life. Observe from afar, don't get too close unless necessary. (Also read 23 Fast Ways to Move On After a Bad Relationship.)
If she's not aware of you and your ex boyfriend's past relationship, then it's normal for her to feel awkward to befriend her boyfriend's ex girlfriend. However, getting close to her can be helpful in case the same things that happened to you repeat with her (though pray that this time their relationship is healthy.) (Also read 60 Signs of An Unhealthy Relationship with Boyfriend.)
Just remember that as worried as you are about your ex boyfriend's new girlfriend, your own safety and health (both emotional and mental) should always come first. If you come too close to their relationship, you might attract something unwanted. (Also read What to Do When Your Boyfriend Gives You Anxiety All The Time?)
If you're still having a hard time getting over an emotionally abusive relationship, there are several things you can do:
This is why you shouldn't use up all your time and energy worrying about someone else. You shouldn't be selfish, but your mental health comes first.
It wasn't your fault—it never was. The way your ex boyfriend treated you was purely all him. The way he acted was never because of you.
Don't be afraid to go to a therapist for help. It has helped many people get over their trauma, and soon you will no longer be living in fear.
At the end of the day, it's important for you to know that his jealous, controlling, and possessive behaviour is none of your fault. He is the way that he is and has his own reasons for acting that way. Just be glad that you no longer have to deal with him anymore.