I've been having this exact same problem recently. I have no motivation to start playing a game, if I do I usually get bored quickly or make decent progress but then lack the willpower to go back to it again later.
I thought that since I was on summer break I'd be playing a lot of games, but instead I've hardly played any. I don't think it's because I'm losing interest in games, but I can't explain what's wrong. It almost feels like playing games, in some cases, has become a chore and not a method of entertainment. This mindset probably started when I decided to try and beat as many games as I could this summer. Now I find myself trying to rush through games to start on the next and that's definitely not the best way to do things.
As for modern games, I don't think I've bought a new title since Skyrim. I've been playing older games but it's not a nostalgia trip or anything like that because I never actually owned most of these games/consoles when I was younger. I think I just like the pick-up and play style that the 8 and 16bit era had. Now if you play a game you're expected to sit down for a least a couple hours to get engrossed, but with older games the action starts right away.