I have sooo BTDT, and am still going through it. PM me if you'd like to talk. One thing you should keep in mind is that he has been thinking about this and researching it for a lot longer than you have. If he makes ANY requests of you, they will not be for anyone's benefits but his own. Don't fall for anything. ESPECIALLY if he asks you not to get a lawyer. Be prepared for him to bad-mouth you to his friends and family. That has been one of the most painful parts for me.
Also, don't freak out if you can help it. This process moves more slowly once you get it going. DO file for divorce and a temporary agreement ASAP so that he will have to continue to support you until the divorce is settled and so that he can't disappear with your baby (as mine thought he was going to do so he could say he'd been involved in her night time care).
Get a good lawyer. I've found my local YWCA to have some great people to talk to. They know by reputation the judges, lawyers, etc and can maybe point you in the right direction.
Yeah, paying for a lawyer can seem insanely daunting, but you will pay for this in one way or another. Either it will be stress, or money. Also, he may get stuck with the bill in the end. You need to be able to focus on your baby and all the joy she brings to you.
I'm so sorry this is happening to you. My STBX left, filed for divorce while I was pregnant, came back towards the end, and then did crazy things until DD was about 4 mos, and I filed for divorce. As a PP said, it's going to be a bumpy ride. Get all the support you can. You're going to need it. You can do this. You will survive. My daughter has been my little angel through all this.