This is not the first time he has behaved like this towards you. And the frequency with which he behaves like this is increasing. ( "often enough lately for me to be a bit concerned it might be a "thing" if that makes sense?" )
"I challenged him about the way he spoke to me and he said "these are facts". "
So, by 'facts' does he mean - "stupid" "a moron" "unfit for any kind of responsibility" "mentally unstable" "should be in a psychiatric ward" and "that my mental state will make me an unfit mother" ? 
OldFarticus this man does not love you. He does not respect you. I would go so far as to say he actively dislikes you, possibly hates you. He is doing everything he can to make you unhappy, going for all your soft and vulnerable spots and hitting you as hard as he can.
I know you say you don't want to leave him. But please look at this situation as dispassionately as you can. Are these the actions of someone who loves you? No. Absolutely, no
. So no matter how much you love him - and I would argue that you do not love him, but an illusion of him - this relationship CANNOT work.
You say it is Jekyll & Hyde. It really isn't. He thinks he owns you, and that it is 'safe' to drop the Mr. NiceGuy act and just be himself. This is who he really is. And when someone tells you who there are, believe them.
"I don't want to LTB but I don't know how to stop him behaving like this."
You CANNOT stop him behaving like this. Nothing you can do or say will have any effect. He controls his behaviour and he is choosing to behave like this. The only thing you can do is to either accept that the rest of your life is going to be this and worse - or to walk. Now, it's always possible that you walking will shock him into changing his behaviour, but it will only be temporary. Remember, this is who he really is. And previous posters have already raised the FACT that abusers ramp up their abuse during pregnancy and once the baby is born. Why? Because it's a vulnerable time and women find it harder to leave in those circumstances. He obviously feels IVF has made you vulnerable enough that he can relax and be himself now.
Please, please, please - stop and think. He's been chipping away at your self-confidence and self-esteem, making you doubt your own judgement. I think you need to get out of his influence and rebuild yourself. I wondered about your mention of 'imposter syndrome'. IMO almost all women suffer from that to a greater or lesser extent, but I wonder how much work he has put in to reinforcing it.
The only positives I can find in your posts are that you should find it relatively easy, in practical terms, to disentangle yourself from this abuser. You do not yet have children with him (tying you to him for at least 18 years), or a house that would need to be sold. Silver linings
. It would be wiser to ensure that this remains the case.