I have the opposite problem, I can't help but finish RPG's even when they're torturing me with insanely long mist labyrinths filled with nigh invincible enemies (cheers for that one, Kingmaker).
Maybe I take the burden of being a hero more seriously than others
. Then again, I might just have been broken by my failure to complete Baldur's Gate as a teenager and have spent my life compensating.
Don't feel you have to complete every side quest as part of a big play through. Do them when you don't have much time and think of it like an episode of a tv show. Something cool that you just go and achieve for entertainment. That's true in The Witcher 3 more than most because the side quests are so good, but can overwhelm the main experience.
The time barrier is massive for a lot of RPG's, I've always loved the idea of tighter but more immersive experiences, but that massive journey is kind of baked into a lot of the genre. I'm kind of torn on whether that time investment is necessary for immersion though.
Weird question, for those RPG's you haven't completed are there any you think about regularly and actually feel are better left unfinished because they can't live up to the hype in your head?