If you put the sentences together as requested:
1) It is often when we are at the absolute point of despair that things suddenly begin to pick up again, and somehow we receive hope and courage and can see circumstances changing in our favor once more; it is worth reminding ourselves that this analogy holds very true in our lives in many ways;
The statement in No 1 is a complete sentence and so the dependent question clause can be added and the sentence still will be grammatically correct.
2) Therefore, never despair and teach your self the inner wisdom that nature shows us, in other words, the truth that the darkest moment is just before the dawn..... It is worth reminding ourselves that this analogy holds very true in our lives in many ways.
'Therefore' is a connecting adverb and this is being used to show a relationship between the ideas expressed in this sentence and the previous one.
Sentence No 2 is dependent sentence and cannot be grammatically correct when given without the previous one and so adding the question clause is only adding another dependent clause and would not make this sentence grammatically correct.
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