Loneliness Support Group
Loneliness is an emotional state in which a person experiences a powerful feeling of emptiness and isolation. Loneliness is more than the feeling of wanting company or wanting to do something with another person. Loneliness is a feeling of being cut off, disconnected and alienated from other people. The lonely person may find it difficult or even impossible to have any...

I am really sorry for what you have been through. Being attacked and abused by someone supposed to be supportive is really not easy, especially when we are not having a good state of mind or self-confidence. Sometimes, we fall apart. And as you mentioned, I think that the hardest part is when people don't believe/listen to us, and prefer to listen to the abuser instead.
Anyway, on to YOU. Having a young child is a great way to make new friends. DONT think about what your ex may or may not have said about you. Don't waste energy on this. Invite one of your child's school friends to come over to play. Then you get to meet the Mum, perhaps you do this a few times, you have a new connection! And it's good for your child.
Being a single parent is tremendously difficult- i' e been one my child's whole life. I never met anyone else to have more kids with. You may or may not, but don't feel in competition with your ex- just because he moved on to someone. Concentrate on a good life for your child and yourself. That's the way to fight back.
Good luck!