Keep a diary and log everything. The new bloke sounds like a smug twat and will try to make you out to be abusive etc, so keep your cool and see CAB asap for your legal position.
Remember to say kind things about her new bloke, even when you REALLY don't mean it. Bitches will often totally dump the new guy if encouraged to keep with the relationship by their jilted ex.
Take the stance that you wish her happiness and stability with him, for her and for the child you care for.
It's actually the healthiest viewpoint for you, and eventually you won't care about the ex's love-affairs anyway, and the sooner you take the 'acceptance' stance, the better - and she'll start seeing you in a better light and be readier to look closer at what she replaced you with!
That would break my heart
Never been in that situation and hope i never am, but i like everyone on here feel for you. No advise sorry just get advise.
Good luck sunshine,