There was a thread the other day about how someone had put down The Witness and found it impossible to return to almost a year later. And it reminded me of how I'd had exactly the same experience, but with Divinity: Original Sin.
The first act of Divinity was tricky because - and it's not a problem the sequel was - there was a lot of quests and information that you had to keep track of. It was a lot of info that could be easily forgotten if you only played a few hours and moved onto other things.
I ended up having to restart a couple of playthroughs just because it was easier to pick up all the information from scratch. And in that regard, it's easy to see how The Witness would be difficult to start over again.
What games have you stopped playing after a period, and found too difficult to start playing again?