OP, I was in your situation many years back: when I received his contemptuous, short message, I sank to the floor with my heart cut out and spent the next couple of years destroyed. Literally a complete shell, barely able to think coherently at all. No amount of vague advice about "keep yourself busy" or makeovers or what have you could have compensated for the simple fact that I just wanted him back. No matter what he had done, no matter what he had said, I wanted him back. Even so, I might have struggled through and got back to something resembling normal if I had not found myself pregnant shortly after his letter and disappearance (we had no children) and with no choice but to terminate. There were no pills in those days, the abortion process was an isolated and laborious procedure provided, if you had the cash, by the BPAS (thankfully not the NHS, the way people's attitudes were at the time).
It was, of course, trauma piled on more trauma, and my mind and body were simply not able to process what had happened. I slipped in a delusional state and started having full-blown hallucinations about the "real" meaning of his letter, "messages" from songs I was listening to at the time, the Virgin Mary, all kinds of stuff, and set off on several wild goose chases across the country, harassing his parents, until a final confrontation got me sort of back to my senses, at least as far as he was concerned. I barely missed being sectioned and ending up permanently mad, and, while I did eventually attend the local psychiatric hospital as an outpatient, received therapy and antidepressants, and was grateful of course, none of it did much to relieve the shock and trauma: only the passing of time and experience of life did that.
Like many others, your experience has really resonated with me, and I don't know if this advice is of the slightest use, but here goes:
Firstly, and as soon as you feel able, get as much hard, vigorous exercise as much as you can and for as long as you can, preferably out of doors. Together with a disciplined dedicated programme worked out by a competent trainer. People say "Oh you must have therapy", but a shock like this is physical: when your heart is broken, something is definitely happening to your actual heart, and, if you think about it, so much physical and emotional exchange with your H over so many years, did actually make him a part of you, so not even remotely surprising that your hair is falling out and you have a chest infection. You don't want to internalise all this until you end up in a permanent state of ill health or have your life-span shortened. I don't know the chemistry of it all, but after you have got into a regular routine of running, swimming or whatever, things fall away, you rise above all the crowded events of our day-to-day life, and past, present and future all fall into proportion: I think we are both more or less the same age, and, after you've disciplined your body to the extent that you can at least partly discipline your mind and emotions, really taking in the trees, flowers and other beautiful things around you, running past the disabled and elderly person barely able to get themselves across the road, you realise that you still have possibly 40 YEARS of precious life ahead of you to do something with through your own agency and your own actions, and not a second more of which should be wasted on your mutant husband or his irresponsible, amoral ectomorph.
I agree with supplements of some kind, but I feel these need to be prescribed, after a thorough check-up, by a really qualified nutritionist, who will be able to deal with issues like your hair: going into Holland and Barrett and trying to choose between the bewildering array of what's on the shelves will do more harm than good, and your body must have got seriously out of kilter after all this.
I also totally agree with PP who advise a self-care programme, but I would go much further: if you can possibly afford it right now, a spa break, immersing yourself in a lot of water for a long period of time, professional make-up tutorials, a micro-needling treatment to pep up your skin, professional hair styling with proper treatments, total make-over. Can you afford, or can you take the time off for, a really great holiday outside the UK altogether, somewhere really beautiful with plenty of beach? (making sure that H isn't able to nab the house or do anything else while you're away). Looking back, I feel that getting clean out of the UK, in a totally different environment for a couple of weeks, might actually have done a lot to jolt me out of the state I was in, and certainly have prevented me from festering in my home, descending slowly into madness.
We're only really guessing of course, but it seems that your husband doesn't seem to have even remotely thought things through, and clearly didn't even begin to think about the logistics and practicalities of the wholesale life change he was about to make: possibly, what happened was that after some ultimatum or showdown with the ectomorph, he came charging round, in his own delusional, dopamine-addled state, imagining that he could just offload the house and get the cash quick for another home just like that (and the market being what it is, selling may well be a matter of years), offload you and the family just like that, and this situation of OW and her husband seems very odd to me: he just does not have the foundations to build a successful alternative relationship, especially with such a woman as she seems to be, and, as PP have said above, what may happen is that he will sink down further in the hole he's busy digging for himself, and his relationship, if you could call it that, will just implode.
In which case, he may well be back, seeking if not a reconciliation, at least a rapprochement with the family: whether you'll want anything to do with him by that time is another matter, but you yourself don't want to be sinking any further down into in the hole he's dug for you. So the answer to your original question may well be yes, but even if he does attempt to weasel his way back, the last thing you want is to be looking like Wurzel Gummidge: you want to be looking transformed, looking busy with some really great project or projects with some purpose beyond all the horrible conflicts and traumas that H and OW have put you through, and with very little time for him and the messes he's making with his life, the house and everything else. That's my two ha'penny worth: it may be bull or completely unhelpful, but I just thought I'd stick it on here anyway.