When you say yes to everyone’s plans, your own plans eat leftovers.

Your own plans are usually the flexible ones, since nobody else is waiting on them and postponing costs nothing visible. That is precisely why they get moved every single time. Anything without an external deadline loses every scheduling contest unless you give it one.

Put your own thing in the calendar with a start time and treat it as a commitment made to somebody. When an invitation lands on top of it, say you are busy, which is true. Doing that twice is usually enough to show how much of your week had been going by default.