You didn't screw your marriage up. You may have made mistakes, I did too. But marriage is suppose to be for better or worse (except in abusive situations)and partners are suppose to work on their relationship. But if one is more interested in running around, or leaving, the marriage isn't going to be fixable. I know it seems hard right now, but your children are going to be fine. Everything seems like its a mess right now but it will begin to work itself out.
AND it is worth postponing the divorce until you have all the paper work you need to finalize. Him giving you the paperwork the day before is not enough time for your attorney to look it over and it include it in the divorce papers. Your stbx is just hoping you won't be able to do anything if he waits until the last minute. One more week or even two more weeks isn't that long. I know we just want it all to be over, the pain, the waiting, and be able to move on. But you have survived so far and another week or two will be worth it in the end.
Teresa is right. Sometimes the key thing is to wait. Even God tells us to wait and stand strong... not to let anger or pain rush us into something. Even God needs time to work things out for us as well. Everything in it's time. I sure learned this throughout my divorce. It takes time to heal, time to think, time to work on solutions, time to overcome blaming ourselves, etc. Don't rush court as Teresa has stated. Waiting does not mean just sitting still and doing nothing, but it does me taking time to let some things work out without your own pushing.