I want to know the meaning of these lines as a whole
Don't worry, even if everything dies around you and it looks like the odds are stacked against you, I will always be with you
And are the sentence structure and grammar correct?
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The meaning of the whole sentence is the last part of it, for it indicates that against all odds that person will always be by your side.
Concerning grammar, I have nothing to correct in these lines. Maybe a dot at the end of the sentence would make it more appealing to a purist, but I am none of them.