Okay so I've just solved this puzzle and have realised, with some hinting from the OP, that the answer here can be obtained in the same way.
In particular, the hidden hint
If we take the first letter of the first line, second of the second,... and the seventh of the seventh it spells UNKNOWN
Hence, I think the answer is
Unreachable, unattainable, always out of reach,
The unknown is something which hidden or not determined and so is, by definition, out of reach.
Unstable, ever changing, impossible to breach.
The amount which we do not know is always changing so what was is now known may have been previously unknown. This changes all the time.
Unkept, unfocused, when you struggle to live.
This, I think relates to the specific unknown of what happens when you die, the mystery of life and death
Unintelligible, with nothing to give.
Something may be unknown due to a shortcoming in communication.
Unpromising, if you leave it alone,
If you do not try to uncover the unknown, it will remain so.
Unanswerable, causing you to moan.
What is unknown does not have a reasonable answer or explanation.
Undependable, impossible to view,
Once something is viewed it is no longer unknown so the unknown really is impossible to view, similar to the idea that tomorrow never comes.
Understand, the numbers and lines are a clue.
This is the hidden hint as specified above