Whatever he says, he will not put you and the children first. You need to focus on what you need and do what's best for you.
Paperwork. Get all the legal stuff together - birth certificates, marriage certificates, passports etc (you can get copies but it is a pain and costly to do so).
Financial. Get all the bank accounts / savings/ investments / payroll / pension / mortgage info secured including any debt info. I would also suggest removing funds from joint accounts although I know this is controversial. If you don't have your own bank account, set one up.
IT. Secure all your IT, change passwords and remove any secondary access.
Work out how you want things to be going forward, housing and financial. Base it on 50:50 - it's all negotiable. Work out where you will compromise.
Prepare your house for sale. Remember it is just bricks and mortar.
Find a good family lawyer and get some initial advice so that you know what your options are (take all your financial info with you, remember time is literally money so keep it succinct).
Buckle up as it will likely be a bumpy ride, but keep in mind why you need to do it and your end goal and you will get through it.
You will realise that you are so much stronger than you think.