I have the same with Diablo 3. Luckily it's easy to get back into so I'll finish the campaign eventually. It's nice but too repetitive to play in one stretch.
I do have trouble getting back to some games. Forgetting some important controls never helps. I took a break from Alien Isolation and when I got back I kept pressing the wrong buttons, not helpful.

Quite often, but not for the games' fault: they aren't the only ones I like, but I like games like Morrowind, Planescape: Torment, Gothic and others that are quite complex, so if anything forces me to stop playing them for a while, and it can happen, I tend to lose the thread, and resuming playing them is often difficult. After I complete the initial effort, usually I can finish the game, totally liking it too, like it happened with the simpler Thief III once the GOG version let me avoid the horrible mess the old DRM on the disc version did with recent versions of Windows, but with Morrowind and Planescape: Torment, despite liking the former and even considering the latter one of the best games ever, I felt lost all the times I resumed them and I started playing other games, pausing them again.

I have a few games I never finished and I know the feeling of suddenly lost interest. Once you are away from a game too long, it's too late. That's why I always try to binge my games as fast as possible. Not rushing but trying to spend as much time as possible with them.

Ka-pi96 said: I used to be the same. But then I got into trophies/achievements and now I'm finishing all my games...eventually

Join our Trophy League then

@OP: No, I'm the exact opposite. If I paid for the game, I'll finish it unless it's a real chore. I'd say I finish more than 95% of the games I buy. With PS+ games it's another story, of course.

Make it like this: to buy a new game, you have to finish one of your old ones.

If you can't get into a game after you've stayed away for a while, play some of the earlier parts to refresh your memory and make it easier on yourself. I find it helps a lot, and I almost always replay earlier missions/chapters/levels, especially the ones I really like, and mess around if applicable, before I've reached the end, even if I haven't played anything else in between.

Maybe it's gaming fatigue, we all go through it every now and then. Even the best of games make you bored.
When that happens, you have to distance yourself for a little while

Right now, I'm playing 3 games at a time (GTA 5, shadow of mordor and Tomb Raider DE). I'm having a hard time determining which game should I finished first. When I start a X game, I start thinking about the other 2, after changing game, then I start thinking again about the last one. It's not easy with limited time. I've decided no matter what I'm sticking with Tomb Raider DE since it has been in my collection since 8th gen started.

You're buying games digitally, no need to have a backlog. Simply make a pact with yourself to buy a new game as a reward for completing the current one. By that time, a game might even go on sale...
I do that since I moved to PS4 and have no more problems like I used to. I just buy one new game each month, and that gives me an incentive to skip less interesting titles.
Right now I'm playing my November game: Far Cry 4. Once I'm done, I'll buy a new one, and since there are no good ones in December my choice is GTA V. That one should last me until Dying Light etc.etc. 12 good games per year is enough for my satisfaction.

You are probably buying too many games. You are halfway trough one and then, bang! New game, let's play it! Stop buying games for a while and work your backlog.

Now, if it's reserved for some games, maybe you didn't like them. It happens.

Give them a bit of time. I remember when I suddenly stopped playing Lost Odyssey at Disc 4 and I love that game. Then, few months later, I get back to it and BLAM! loved it again and finally finished it.

I think that you might get burned for playing too much in a short timespan. How much time you dedicate playing these games?