Ah yes, spiraling into existential dread... My favourite hobby hahaha.
But on a serious note, this is a good question and I've considered it a lot. It's difficult to genuinely claim all our thoughts as our own because we rarely (or at least, I rarely) have conscious control over what pops into my head at any given moment.
There are of course times were I'll sit in contemplation and have conscious control over my mind and I am intentionally creating or working through thoughts. But even then, sometimes an idea or new perspective hits me out of nowhere, and it's like... I had nothing to do with that. It simply arrived, like a gift from the universe.
Do we have ideas, or do ideas have us?