Play it cool.
See CAB asap for immediate advice - i.e. new tax credits, etc. Get a list of recommended lawyers from CAB, and choose one that specialises in family law, legal aid, and gives free initial session. Make appt. asap, and get advice - this is vital. You will feel more in control to be doing this practical stuff.
Tell at least one or two real life friends/family and lean on them.
Don't (and this is really hard) contact husband, don't talk to him, don't text him. Right now, the grass is greener - he can't be allowed to have the best of both worlds. He can't know that you want him back - (even if you do). Don't give him an option. Often, these men think the grass is greener, but actually moving in the OW forces them to be themselves, and the OW may well not want to be with him full time. They may have a "honeymoon" period, but as well all know, at some point this wears off and reality sets in. Let him think that you are using the time to decide what you want. And this is what you need to do: focus on what you want. Take as long as you need. Cry, get angry, get strong....and reflect on what you want. The thought of being on your own, is often much worse than the reality. He wants out - let him have what he wants, and detach from him. He will miss the kids and your unconditional love and support. But he has to miss this fully, without you being there as his safety net.
Only have minimum contact (preferably text) about seeing the kids. And until you get advice from sol, this will be as you want it to be.
Take it an hour at a time, a day at a time....and focus on yourself.
Keep posting - you will get through this. There are loads of us who know how you feel - and we can help you. And you will, I repeat, will, cope with this.