I understand, I'm having a family crisis myself. in these times all you want is those controllable things around you to go back to 'normal' to try and help you through the health crisis that engulfs the family.
The trouble is, these things are always going to come up. Our families are mortal and it's rare there's a time when no unexpected difficulty assails us and breaks our heart when things are bad.
I really really think you should address some sort of counselling option. Without it you are rudderless and structureless. Most importantly, you must be able to see, if you step outside your situation for a moment, that trying to suppress your own totally valid fears and weakness in order not to frighten him off will always end up being counter-productive. Without wishing to hurt you further, he already has risked your marriage, he already has countenanced 'leaving' it. He probably is oscillating in his head and there's nothing you can do about it. If you were perfect in every way at this point it wouldn't stop him doing that. He's gone from 2 loving women to 2 vulnerable broken ones. He is selfishly dealing with his own pathetic loss. He is already way way ahead of you emotionally and is going to feel stronger than you as a result. To look down at your weakness or find it unattractive at this point is just selfish and manipulative.
Love is all well and good, but it should be our servant not our master. Every woman in an abusive relationship begins the reasons to stay by saying, 'But I love him!' It is sadly, absolutely irrelevant. Read 'Why does he Do That?' by Lundy Bancroft and see if it speaks to you. The other thing women in relationships that are bad for them always say is 'If you knew about his childhood you'd understand.' NO abuse, however awful, means that you should end up on the receiving end of some eventual consequence of it. ALL men who mistreat women, in what ever way, have had problematic upbringings. It makes no difference. It is okay to be angry with him rather than frightened. Write a letter, then decide whether to give it to him.
You are quite right that he is the one that needs to put the effort in to the relationship, but you are the one who needs to put the effort in to you. With every thing you do for yourself and by yourself, the stronger you will become, the more you will detach from him. Talk to friends, invest in your health and work. Find as much self-esteem and support and interest outside of him as is possible, and let him come to you with the ideas of how to make the 'couple' strong again. And believe me this does not mean he will become less interested. Quite the reverse as every single thread on this board will attest. The stronger, more independent and detached you are, the better catch you are. Men often bewail that quality in women when they have dragged it away from them. A man who stays with you only because you are vulnerable and he feels guilty will not stay long, anyway, I promise.
Of course you feel physically ill with tension. It is normal. Often likened to being hit by a bus.
Also, it is not YOUR problem that you doubt he really wants you. It is not your problem to solve. He has to solve it. If he can't convince you then it is HIS fault. Not yours. Either he does want to fight for it, and will reveal that in time, or he doesn't and is looking to escape. Either way your strategy is the same. Look after yourself, get yourself strong, put the onus on him. But communicate. Make demands like counselling etc which are MORE than reasonable in the situation.
Start running with smaller goals, running a bit, walking a bit, wear an app or headphone music to blot it out a while.
The blame you are laying on yourself is not deserved. You have been through a trauma, like losing a loved one. It is okay to tell people. He has acted. These are just the consequences.