Trying to leave

[ZingyOpalRaven]

I posted earlier on about leaving a relationship. I have told my partner that we aren't together anymore and he wont accept it. What am I meant to do, we have a baby and a mortgage, but I feel very trapped now

What do you mean "he won't accept it"? Is that he is refusing to move out? If so, What's your mortgage arrangement? You might need to seel the house of one of you can't buy the other one out.
Otherwise, I am not sure what else it could mean. He can't have a unilateral relationship.

He keeps saying he's going to prove that he will do whatever he can to keep us together even though I have told him not to.

Tell him you're going to see a solicitor and file for divorce. Then do it. He can say whatever he likes but he can't actually stop you divorcing him. Unless you feel unsafe, in which case keep quiet and get legal advice and support from family and friends to leave safely.

Just keep saying no. It's quite normal for people to make big declarations. It's quite a shock when the partner is leaving the relationship. All you need to do is to stay the course and make sure he knows that that's what you intend to do.