For those who married or settled down later in life, what was the hardest part about transitioning from the ‘single mindset’ to the ‘partnership mindset’?

If you spent most of your 20s or 30s fiercely independent, buying what you wanted, and making decisions entirely for yourself, how hard was it to genuinely adapt to sharing a life? What were the minor friction points or mental habits you had to break that no one warns you about?"