Should I have said "of my life" or "in my life" in this sentence:
to build meaningful relationships in important areas of my life
"Of" is the correct word there; "In (qualifiers) of (item)" is the normal construction. To use "in", you would lead in with "at": "At (qualifiers) in (item)." (To give a concrete and relevant example, your sentence fragment is using the phrase "important areas" as the qualifier for the main item, "my life".)
Often, "in/of" is used for broader, sometimes vague groupings, whereas "at/in" is always used for more specific things: "in some parts of the world" vs. "at several points in the process." Since your sentence uses the pretty broad and vague phrase "important areas", using the at/in combination would not be appropriate.
Another way to view it is that "in/of" is used to express containment, while "at/in" is used to express location. Your life clearly contains several important areas, but they are not physical areas that you are trying to locate on a chart or diagram, so "in/of" is the appropriate construct. On the other hand, if you are speaking of your life metaphorically as a timeline, you can certainly say something like "I was at a critical juncture in my life" to reference a specific, pinpointable moment.
Of my life is the correct usage.
'Of' means 'belonging to or connected with something or someone'. In this case the 'important areas' belong to 'my life'.
'In' (in relation to 'my life') refers to something or someone 'within' your life, but separate from it:
You would never say:
Either is acceptable. "Of" is probably more idiomatic, but I don't know on what grounds the other repliers have asserted that "in" is less "correct".
In the BNC, "of" gets 5 hits, and "in" 0; but in Google, "of" gets 6.74 million and "in" 1.1 million.