You grow old and die anyway. And then there's everything you have to endure in between/inbetween: grief, loneliness, sickness.
What's the correct choice? And why?
Between on its own is a preposition while in-between functions more like an adverb.
We usually use between before the object it refers to:
Between life and death is a long, hard life.
In-between normally comes after the thing it refers to, like in your example.
Here is another example using in between:
I couldn't find my key anywhere, not under the sofa or the chair next to it. Then as I stood up, I saw it glistening in between.
In between (2 words) is the correct punctuation for an adverbial phrase, which is how the phrase functions in your sentence. It's also the correct punctuation for a prepositional phrase (e.g., "I live in between two skyscrapers"), but that usage is unnecessary since you can just substitute "between".
In-between, with a hyphen, is used as an adjective ("an in-between realm") or a noun ("neither here nor there but the vast in-between"). These usages are an order of magnitude less common than the adverbial phrase.
Inbetween, with neither a space nor a hyphen, is non-standard, as far as I know.