A lawyer will help you immensely. Any layman knows that in a marriage with kids two major things happen during a divorce. The marital assets are divided and custody of the kids is then arranged. A lawyer is your guide through the legal system and will, hopefully, make sure that you get what you are legally entitled to out of the marriage as far as the assets go and as far as custody goes. I think part of the issue is you are still emotionally invested in keeping the marriage going, and keeping your life within that marriage alive. It is my experience as a friend to many divorced persons that once a separation is being talked about and once you are sleeping in another room where once you slept together, the marriage is at the stage where it is effectively being kept alive on a ventilator but death is imminent and cannot be stopped. I’m sure there are wonderful stories of marriages going to this point and rebounding, but never in my life experiences has it happened (hell, I’ve heard of people getting divorced then remarried, but I think that is very rare.)
To a degree you’re going to need to get emotionally unattached from the marriage as best as you can, even though doing so completely will of course be impossible. You have to start thinking about getting what is equitably yours and getting the custody arrangement you want, and to do that you need a lawyer, your focus needs to be there, not on fixing the marriage (you can’t bring back the dead.)
I may be able to dig up a link later, but right now my searching is not going well using the forum’s search feature, however I believe I have read from one of the lawyers who posts on the SDMB that shared custody has become the norm and the system where one spouse got primary custody and the other only got visitation is not prevalent like it used to be–but I have generally little interest in such things so I’m just working off of vague memories. So even though you spend more time with the child you may end up with some scheme of shared custody unless you can convince the wife that you are their primary caregiver and that it is in their best interest for her to give you sole custody w/visitation rights (either way she should be a part of their life.)