Hi imcd72,
sorry to hear about your situation. I'm sure that there are many on this site who have found themselves in similar situations.
Once, you have got over the initial shock, you must be practical. Having read through your post, you are in what is commonly the female's situation. As has been stated on here, the law leans towards helping the main carer for the children - normally the female, but in this case you.
It will take you ages to get over the relationship. As Fiona stated, it sounds as if your STBX is ahead of you in that. But you must move on a little.
If you have not yet got any legal advice, get some! It's a very hard first step, but crucial. You need practical advice from someone who is not emotionally involved. I would definitely NOT move out. You have as much right to occupy the
FMH as she does, so stand up for yourself. Many solicitors have a free first session, so cost initially should not be an issue.
As usual, the kids are the innocent party here, most in danger of being badly affected. Unfortunately some people like your STBX have little consideration for them. But you obviously do. The kids will realise that.
Get some legal advice, stay in the FMH and make a vague plan as to what you want to do for the future without your STBX. You need to accept that this relationship is over sooner rather than later - a bit like having a death in the family - so that you can 'move on' to new better times.
There is a wealth of experience and knowledge on this site, from people who really do have a clue. Use it and I hope things improve for you. Don't expect it to be overnight, but a bit of progress will make you feel a little better.
Good luck,
Anthony