I'm 8 months ahead of you with a cheating husband so here is my advice in no particular order. This isn't about DB'ing it's about protecting yourself. I'll leave DB'ing to the veterans.
1) The advantage to filing first is that it puts the other person on defense, you get to go first in court and you control the process. If you want to postpone you can. You have time to plan, to shop around for an attorney, to pay the retainer, and fill out the paperwork. Go to court if you can and watch litigators in action. Find an attorney you like and schedule a free consult. The attorney works for you so if they push you to file, settle for some every other weekend custody walk away. I did all of these things but file 3 weeks after WAH left. When I was ready to file all I had to do was text her. She emailed me the complaint the next day to sign/date and it was done.
2) That said you are not ready to do anything until you have a plan. You can count on that she is already planning and that she is 10 steps ahead of you. You need to catch up. Cut back all spending, cut, cut, cut take back control of the finances (if you can). Live as if you are divorced NOW. Hope for the best prepare for the worst.
3) Your number one goal is custody. Focus on that first. Keep a journal and document all the time you take care of the kids vs when she's gone. Be super dad and while you probably won't need this it is better to be safe than sorry. Do not let her see you doing this.
4) Your kids are little and while it will still be hard they will adjust quicker than older kids. I've got teenagers and I'm about to have to put my 17 year old on anti depressants. I know now that WAS's don't care about anyone but themselves. You can't nice them back, guilt them back and they don't really care who they hurt as long as THEY are happy. They will (sometimes) use the part where you want to "nice" them back to their advantage. Don't fall for this. No one ever came back because they got a great divorce settlement. They come back when they are broke and their new life stinks.
5) Do NOT move out of the house. Ever. If she wants out she can leave.
6) Gather all financial documents and put them in a safe place. Might be harder to get them later. I did this in week 2. He has yet to ask for them so I have successfully caught him by surprise. I have everything. Birth certificates, car titles, tax returns, credit card statements, everything. If there is anything you want out of the house take care of that now. My own ex best friend moved out while her husband was at work.
Look you sound like a great guy and I pray you won't need any of this advice but do it anyway. You'll regret doing too little but you won't regret doing too much. I stayed in limbo for 5 months because that was when the lease was up and he forced me to move. At that point I knew I'd probably have to file soon. I filed in month 7.
