With lyrics like “all my darkness comes when I close my eyes / I’ll be seeing you in the next life”, it could be a mean a person with a generally sunny disposition having dark moments only when they fall sleep, and waking up brings a fresh start in a new day, hence “the next life”. It could also take on a darker meaning, which would bestow a “yin-yang contrast” to the light, cinematic feeling of the musical arrangement. 

Overall, “I’ll Be Seeing You” could also make you miss someone you fell in love with who you’ve never had a romantic encounter with. You’ve developed feelings, and have come to realize they have a totally separate life you don’t fit into at all. Enveloped in sadness, you try to kill off the feelings, salvage your broken heart (re: “digging up a broken heart”), try to sleep away the pain, to wake up to “the next life”, as in “the next chapter” without the feelings.