To deal with the darkness outside, you must first deal with the darkness inside. One of those pretty sentences, that seem to offer so much insight and illumination (pun intended), and yet actually lie pretty thin on the ground, hiding behind the elegance of the language. Often this type of writing is praised, and from a beauty of language point of view, I understand. From the point of view of furthering human understanding, I'm not sure it adds very much. Many people who put pen to paper, or finger to keypad, are lauded for this kind of thing, when in fact they are simply not criticised because readers feel intimidated and unsure of the meaning of the words, but instead of thinking their lack of understanding comes from a fault of the writer, they take it upon themselves and assume their own ignorance or lack of mental capacity.
So let's unpick it. One reading would be that it is essentially saying that it is harder (not impossible) to deal with trouble outside of yourself, in your relationships with others and the wider world, if you are troubled within. Well, if you are troubled by things inside, your judgement or ability to see things objectively (as objectively as any of us can from our by-definition-subjective view point) will presumably be hampered because you are coming at the thing outside with so much more baggage. Baggage is a weight to carry, you lean into it, and away from the direct view.
Another reading could be that the darkness outside is in some way caused by the darkness inside, therefore deal with what it is inside, and you simultaneously deal with what is outside. Two birds with one stone and all that. Or in fact there is only one bird in the first place... And what is this dark bird? This darkness? 'Trouble' as I said before? Conflict? Confusion? Unhappiness? Anything that doesn't feel 'right' or good? Perhaps simply dark as opposed to light...un-illuminated in contrast to illuminated.
And how illuminated by these words do I feel now? Not very. They sound nice, but the intended reading is not clear. Beware dark, unclear words.