grammaticality - Why use "that" instead of "the"?

I think it would depend upon context. "that extent" implies to me that this is referring to old information. For example, the sentence before might have been "Once you find someone you love, you must trust them utterly." As you noted, the second usage sounds right without context because you mentioned extent earlier in the sentence and therefore you can refer to it.

Personally, I'd put a comma before the ampersand (well, and I'd spell out the "and") to clarify that it's not the person feeling guilt for caring. It becomes clear on a second reading, but in the first readthrough, it may require a minor bit of backtracking to make sense of the sentence.

• Next formality - How often do people say "gotta", "wanna" or "gonna" in English speaking countries?
• Previous prepositions - Passive voice of intransitive verbs