I think the quote is either fictional or very obscure 🤣
The author of the quote is 「ディオクラテス」, whose closest romaji is "Dioklates" or "Diokrates." which doesn't have an exact match. When you Google the name in Japanese, the only reference is to a vacation blog post about Emperor Diocletian, talking about hispalace in Split, Croatia. I can't find any quotes by Emperor Diocletian anywhere close to the quote so I think that's out.
Meanwhile, the closest thing I can find to the quote is from an op-ed piece from 1942 by Tokuryuu Yamauchi:「さうして分たれたる運命は自己の中にあり自己によつてあるものでなけれぼならぬ。」It's pretty close, I think?