Okay...this may be long...I have been where you are too. It's not an easy road, but, it IS do-able.
My kids were 9 & 11 at the time. We had been together 20 yrs.
Importantly, you need to get a good solicitor. You need a separation agreement drawn up. Your Sol will draw up the proposal and send it to his Sol. I realise that neither of you probably have a Sol just now. Ask around for recommendations, when I was going thru it, a friend had recently divorced and recommended someone. They will know things you are entitled to, that you have no idea about.
Eg.
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If his pension is currently valued at more than yours, when the house is sold, you get more of the proceeds to even this up.
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You may be able to stay in the marital home until the 4 yr old is 18.
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If you have sacrificed your career whilst he has progressed his, you will be due more money from the house sale proceeds (it's called economic recompense)
It's hard because you are going to have to sit him down, and tell him that you want to separate and then do all this Sol stuff whilst (at least for a while) living under the same roof. I told my ExH in the February and had to live under the same roof until the August. Hell on earth, but you have to see it as a passage to better times.
To answer some specific questions you have:
Can he stop me? No, he can't. He might turn really nasty tho. My ExH assaulted me twice, which was fairly out of character.
Will he get 50/50 custody? Well, yes he might. You have to sort of bash that out between you. It's not easy, because you will miss your children. You have to alternate Christmas and special occasions. He will want to take them abroad without you and stuff. That's not easy at all.
Be prepared for some shit to fly. My ExH came up with tons of weird shit, like suggesting that his parents get a loft extension and the kids could live with them now (300 miles away). Er, what? Also, he tried to stiff me on child support, the split of assets....he stooped so low I was gobsmacked.
Stick to your guns. Soldier on. You could be free by Christmas.
Fwiw, I went on to meet a lovely man. We are now married and will celebrate 10 years together next year. It's yours for the taking.
I remember constantly listening to inspirational music at the time, to get me through it. I will try to remember some and post here for you. They might not be your cup of tea, but it will at least be a distraction!