So your character has made a momentous decision. He/she is leaving their partner. They have decided to quit their job or move to the other side of the country or leave their children with no intention of ever seeing them again. That's sorted then: decision made and onto the next problem.
Characters are designed (hopefully) to be as realistic as possible. If you suddenly decide the person you love is no good for you (perhaps because they're a criminal or an expert manipulator) then you might make the choice to distance yourself but in reality it is not simply a case of drawing the line and sticking to it. We human beings constantly analyse our decisions and most of us dwell on the past at regular intervals. If your character leaves their children in chapter three then they will return to it in their heads at a later point - unless they have a very good reason for not doing so.
Of course, your viewpoint plays into this. If you have a first-person narrator then you may have to be a little more explicit than a third-person narrator would have to be when describing the after-effects of such a choice. Demonstrating internal conflicts can be an integral aspect of first-person narration and it's something that shouldn't be forgotten.
The key point here is that you must be prepared to deal with the consequences of killing someone off, moving someone out or making an altogether different life-altering choice. It cannot ever just be a means to an end. If your protagonist ditches his wife in chapter one and goes off to search for oil without ever mentioning her again, you have to wonder whether he should have had a wife to begin with. If she was simply a springboard for the main storyline to begin then maybe she could be replaced with something else which wouldn't baffle your readers when they failed to hear of it again.
It is irritating as a reader to come across something which you think will play a vital part in the ongoing plot only to realise it was a device that was utilised then abandoned. When that happens I lose faith in the author; not the book, because I've usually finished it before I realise what has (or hasn't) happened. But it would make any repeat business from me unlikely.