I've asked before and if it's been answered bear with me. The linked story here comes from the Washington Post, but the issue was reported on by many news outlets:
Trump admits to Russian hacking even as he attacks U.S. intelligence community
What changed Trump's mind?
He dismissed the claim that Russia had compromising information on him as "fake news." But something must have been said to him, or shown to him, that led him to finally concede Russian involvement. One explanation is that he was shown or told about Russian surveillance of Trump himself. I can't imagine anything else changing his mind. Digital footprints wouldn't have done it; it would have to be a smoking gun, IMO. Something simple that Donald could understand. The existence of video he hadn't known about, or possibly even the video itself, probably would have done it. But has he or anyone else ever said what changed his mind?