Solo (strange calling someone else that :)) - I haven't read your whole sitch. Have you seen a solicitor yet. If not, do so immediately. Most will give you the initial consultation for free. Use the first session to learn the process and also establish your rights. You don't need to commit. In fact, if and when it goes down that route, I would recommend meeting with a few until you find one that you are comfortable with. But absolutely go and meet with at least one. He is asking for the marriage certificate because in the UK, you cannot initiate proceedings without one.
As many here have said, please stop sleeping with him. I know that at the time it feels like a connection (and it is) and you so desperately want to feel a connection, but he is literally having his cake and eating it too.
Fathers day - I offered my H the kids on fathers day too (it is my day) and he said he is working (which is probably true). So, I will make plans with the kids without him. They will call him on Sunday and then on Monday when they see him give him the cards and the present that I bought. He is their father. He has not stopped being their father. On fathers day he gets a card and a present.
My H comes from a broken family where partners were introduced willy nilly. He would not do that to our children - but if he did, unfortunately there is nothing I can do about it. I could try and reason with him but ultimately, he can do as he wishes. A friend tried to have it put in her separation agreement that they would not introduce partners to their children for at least a year after he MO, but he refused to sign the agreement so she had to take it out. The best thing to do IMHO is to show your children you are OK with it. So you do not burden them with your pain. They don't need that. It [censored].
From what you described, moving out was the best thing he could do. Living in an environment like that stifles your energies. It is a black cloud hanging over your home - the place you are supposed to get rest, and respite and escape from the rest of the world. There was no escape for you or for him. It would not have been good for your children either. Now, you can start to clear both your mental and physical space ... so you can think. Not just react. But really think about what you want.
I am not saying it is over. It isn't over until you say it is over. But right now, I don't think you are seeing things clearly enough. There is too much noise.
