I too find it hard to finish most of the games I play. I mostly 'blame' this on the arrival of my two little ones and not having the time I used to.
Being a bit of a night owl, I do still get a decent enough amount of playing time and these are the titles I've been struggling with because I know I like these games and they're not bad games by any stretch...
Fallout 3 - I know, it's a classic, superb game but I just cant get into it. I enjoyed the start of the game. Found the dialogue feature fun and looked forward to hours of fun. I've barely made it past the first couple of missions. I don't know why?!
Borderlands 2 - Played the first game to death cause it had that re-playable magic that some games have. Took all four characters to their peak levels and just kept going back for more. For B2 I completed the main game once, with one character and I'm bored with it.
Dead Space - I think my main problem with this game is that everyone told me how scary and atmospheric it was. Despite playing in darkness with my earphones set to freak-out I never once felt 'the fear'. I found ResEv4 + 5 to be scarier and nothing has come close to the terror of the early Silent Hill series.
As for re-playable titles, I used to play a lot of Football manager up until the 2012 release and in terms of hours put in nothing comes close. My main go-to game now though is Company of Heroes and its Mods. I discovered this game only a few years ago and I have been hooked ever since.
Roll on CoH 2!