Really sorry this has happened to you. Like Counting, I think it's best to let the shock and emotions come out before dealing with practical issues and finances.
I think in all probability there IS someone else for your DH. How would that make you feel?
You've been very honest about what's led up to this - do you think there is any chance of those "in love" feelings returning in you, for your DH?
Does you heart break about losing HIM or at losing the "family set-up"?
If you can honestly say that you aren't traumatised at losing him as a person, then really, he's been the brave one here, asking to leave. No-one should live in a marriage without love and passion - and that includes you.
I wouldn't rush to counselling until you know the full picture though. IME men never leave without having someone waiting in the wings. Sometimes it's a relationship that has already started - and sometimes it's for a relationship that they want to start. Many people delude themselves that they are being ethical leaving a relationship before committing a physical act with someone else, but the truth is far more complex. If he's let someone into his head - he might as well have let them into his bed.
I hope you've got RL support. In your position, I would carry on being as fair and non-judgemental as you sound, and sit down with DH and ask him to be honest with you. If there is someone else, it's much better that you know.