Switch things up often between genres/ consoles/ generations.
I find that when I spend too long on any certain genre, I start to feel the burn out too. So to keep it interesting, one week I might be playing contemporary releases, next week I might be dabbling in my Japanese PS2 exclusives or tinkering with my Supermodel emulator and enjoying some classic 90's Sega arcade games. Sometimes, I might even go one or two weeks focused on a certain franchise that spans several years and consoles while squeezing in some online fighting game match sessions. Once or twice per month, some of my coworkers and I play some GTAO together too. It probably sounds like gaming ADD, but I just don't feel like committing myself to any specific game or genre. Which in turn keeps things interesting.
As a result, it obviously takes me months, if not years, to finish the long SP games. I'm also, as they say, a jack of trades but master of none since I don't really invest a lot of time on any one game. But it keeps my gaming sessions as a fun novelty to look forward to rather than a chore that I procrastinate.