I know that feeling. Between Rare Replay, Carnivores 2, Fallout:New Vegas, and The Taken King, with Fallout 4 and Doom on the way, my time is going to be very split.
Besides, with all my characters at max rank, there’s a lot less for me to do now 
I think you have to ask yourself a few key questions:
1) Do you enjoy the gameplay? At it’s core, most basic level, do you have a fun time playing the game either by yourself or with your nephew?
If not, then the simple answer is that while you may want to like the game, it might not be something you can force yourself in to being interested in.
I had the same experience with Mortal Kombat X. I was really psyched for it, as I am for every MK game, but when it came out, I played it, beat it with all of the characters, beat the story and then was left with the rather stunning conclusion (to me, I mean), that I simply didn’t enjoy it all that much. The character variations felt like a gimmick / half-measure, the unlockable content was uninspiring, the story was pretty weak, but the gameplay itself… The meat and potatoes, just felt choppy and kinda dull.
I still play it every now and then, but I’m never overly enthusiastic about the idea of popping it in and playing a few matches. Slogging through an entire ladder in single player feels like a herculean ordeal. Sometimes, that’s just how it goes, even for games you want to like.
2) But if you do like the gameplay itself. I’d ask you this: How do you feel before playing the game?
Sometimes, playing KI can honestly feel like work when you’re trying to grind a character up to level 50 and unlock all of their accessories and colors and what not. That feeling tends to set in for me sometimes when I’ve played three or four matches and I see my XP meter slowly amble up toward the next level at a drunken snails pace.
Personally, I think that the grind is a bit too punitive. Or at the very least, it’s geared too far in the direction of hardcore players that tend to main one or two characters, as opposed to people that simply want to unlock all of the character appearance content.
I’d love it if they somehow divided content between “base level content” (ie character appearance items) and “prestige level content” which could be non-character appearance items that hardcore players would actually want to earn and show off online, but that casuals wouldn’t feel like they’re missing out on if they don’t main one character and use that character to the extent that hardcore players use them.
3) So let’s say you enjoy the gameplay and you initially enjoy the idea of popping the game in. I’d have to ask… Do you get this way when it comes to other games? Do you buy games and then don’t play them for days or even weeks after you purchase them? Do you play a game you were looking forward to a good deal at first, but fall out of it easily, and the idea of going back just sort of floats in your head like “yeah, I’ll get back to that some day?”
If this is the case, then I think it could be one of two things… Either you’ve inundated yourself with too much content (both in games and in life) or you might be burnt out on games in general.
For the first one, well… I played a lot of games when I was younger. At the arcade, at friends houses, but I didn’t have a video game system of my own for a long time. So when I finally got one, and started making money to where I could afford a bunch of games, then two systems, a bunch of games, then three, then four and so on… I couldn’t get enough.
Until I got enough. At some point, my life got busier, and while I still love sitting down and playing a game a few times a week, I don’t consume them the way I used to. I simply don’t have the time and can’t stay up until all hours of the night like I used to.
But not only that, I more or less gave myself a content cavity. Too much of a good thing. I now have a decent sized library of games and systems, and sometimes I almost feel guilty for not pulling a game out and playing every now and then. I get games, whether classics or new titles, and sometimes I don’t play them for weeks, months or at all (though that tends to be classic titles that I want to own for my collection).
Does that mean I’m burnt out on games? I don’t think so. I still enjoy games. I’m just still trying to reconcile the fact that the way I consume them no longer matches the way I consumed them when I was a young kid, and that’s okay. I just need to inform my wallet of that fact and maybe try and stick with one game, finish it, and move on to the next. Be more focused.
Of course, that can be hard with a fighting game, when there’s really no “end” that you can move on from. You basically just decide that you’re not that in to it anymore and you move on.
So yeah, maybe just do a little mental inventory and decide, is it this game only? If so, how do you feel before and while playing? What makes you want to put it down? How have you consumed games in the last few years? Has that changed at all?