If you're who I think you are, you were advised to make him leave when you first discovered the extent of his duplicity.
If you had acted on that advice, I reckon you'd be further along the road to discovery recovery by now - and it could be that the salutory shock of being turfed out on his ear at the time would have enabled you to prise the truth out of him some months back.
Would you have any objection to linking your earlier post as the history does need to be taken into consideration by those who are not familiar with the story?
You know my view, and that of many others, which is that he's been getting his leg over with the ow throughout the conception, births, and hospitalisation/illnesses of at least 2 of your 3 small dc.
It seems probable his first dw turfed him out for infidelity and the reason why he won't come clean with you is because he knows the truth would be the end of him as far as you're concerned and it would be, honey, because you are an honest and upright individual.
Tempting as it may be for you to think that you could work through his inidelity with a view to having a stronger marriage, you're the type of woman who can't abide a liar and any love you have for him would rapidly wither into contempt - as it is doing now.
When you first confronted him, his reaction was anger and he physically assaulted you. At no time has he given you any details about the ow such as her name and address but, to the best of my recollection, he has varied her age on at least one occasion.
Since you found the mobile phone he initially claimed was his mate's, you've had a cock and bull story about how he and the ow once worked together or some such and she made contact with him, plus a load of twaddle about her being someone he could 'confide' in - as if you weren't ready, willing, able, and on hand 24/7 to listen to his 'confidences'.
Sorry to say, honey, but your marriage is dead in the water and I suspect the duplicitous twunt is still in contact with the ow.
Have you made it to a solicitor yet?