"I'd rather have less than be behind"
Now the belief that keeps the whole thing going, and I want to say at once that it isn't a flaw in you. It's how people are.
Sara Solnick and David Hemenway, in 1998, gave people a set of choices between two worlds. In one, you have more of something than everyone else, but less in absolute terms. In the other, everyone has more, but you have less than the others. For income, the choice was between fifty thousand a year where others earn twenty-five, and a hundred thousand where others earn two hundred. Half the respondents chose the first. They'd take half the real income to be ahead of the people around them. The same pattern held for other things, strongest for attractiveness and for praise from a boss, and weakest for holiday time, where most people just wanted more.
That was a survey, and people's choices in surveys aren't always their choices in life. But the finding is one of the best known in this field and it's been repeated in various forms. Concern about relative position is normal. It's not greed or insecurity. It's a preference that half of people will state out loud when asked plainly.
So when you say you'd rather have less than be behind, I believe you, and I'd point out that this is the mechanism, said in your own words. Keeping up is what a normal preference for position does when a table is in front of it. The kitchen isn't bought because you needed a kitchen. It's bought because next door's is newer and the preference for position says that's intolerable, and the preference is real, and the kitchen is twelve thousand pounds.
Here's the question I'd ask. Position relative to whom, and for what? The preference in Solnick's survey was about income and attractiveness and praise. It wasn't about restaurants. Nobody at the table is ranking you by what you ordered. The position you're protecting with the split is a position in a ranking that isn't being kept, and the preference that drives it is being spent on a thing it wasn't about. You can have the preference and decline to feed it at the table, and the next chapters are about why that's safe.