When you kept up again
It'll happen. You'll be at a table, tired, and the split will come, and you'll pay it without a word, and only in the car will you remember there was a method. Or Lisbon will arrive in the chat at eleven at night and you'll have said "count me in" before you've read the second message.
One split isn't the month. The list will show a month with more asked prices in it than any month last year, and one split is one line on it. What happened is that the table is fast and the habit is old and the sentence didn't come in time. It'll come next time, because you noticed this time, in the car, which is sooner than you used to notice, which was never.
Ask the price late. It still works. Find out what Lisbon comes to, now, after the yes, and hold it against the wanted column. If it's not yours, "I've had a look at the dates and I can't make it work, have a wonderful time" is a quiet no that's available for about a week after any yes, and nobody remembers the gap.
Work out which table got through. Was it the late-night chat, where you say yes before reading? Then plans that arrive after nine get answered in the morning. Was it a particular friend whose enthusiasm you can't decline? Then the question goes to them first, before their next idea. Was it the split, at the moment of the card? Then the sentence gets said as the card comes out, every time, until it's habit.
And notice the size of it. A split paid without a word is sixty-eight pounds and an evening. It's not a return to the old month. The old month had nineteen of them, and this one has one, and the one is on the list with a note beside it that says "tired, work dinner", and that note is the difference between a habit and a slip.
No streaks. Don't count evenings where you held the line. Count prices asked and let one unasked split be one line in a list that's mostly asked. The month's total goes up a little that month and down the next, and the direction over the year is the only number that matters.