This sounds like it could be Belgarath the Sorcerer by David and Leigh Eddings, prequel book to the Belgariad and Mallorean series.
When the world was young and Gods still walked among their mortal children, a headstrong orphan boy set out to explore the world. Thus began the extraordinary adventures that would mold that youthful vagabond into a man, and the man into the finely honed instrument of Prophecy known to all the world as Belgarath the Sorcerer.
Then came the dark day when the Dark God Torak split the world asunder, and the God Aldur and his disciples began their monumental labor to set Destiny aright. Foremost among their number was Belgarath. His ceaseless devotion was foredoomed to cost him that which he held most dear--even as his loyal service would extend through echoing centuries of loss, of struggle, and of ultimate triumph.
All of the facts fit from Belgarath's lifetime of several thousand years to him and others being servants of the god Aldur, to his brother Zedar betraying Aldur and stealing the orb for Torak.
The Wikipedia entry notes that it is indeed written as an autobiography:
The book opens shortly after the end of The Malloreon with Belgarion, with help from Durnik, pestering Belgarath to write an autobiographical account of the events prior to The Belgariad. The core of the book is in the form of Belgarath's memoirs starting with his becoming an outcast from his village and becoming first disciple of the god Aldur and ending with the birth of Belgarion—a span of about seven thousand years.