No-one has a good understanding of how the brain will function for extended periods of time. But there are some things known.
The brain, despite everything, does not have infinite capacity. As your lifespan extends, the memories of what are less important blend together, collating down. Similar experiences all merge into one haze, the first time you experience something sharper than the rest but still joining together.
Repetition keeps certain activities fresh in the mind, but there is so much you will not remember.
And when you have so many things to remember, your mind itself will take longer to find what you are thinking about. Becoming lost in your own thoughts, for minutes, hours, days, as you struggle to place faces, to recognize landmarks that you have seen so many of.
tl;dr a million years? Anyone would - to our standards - be utterly insane.