Unreachable, unattainable, always out of reach,
Unstable, ever changing, impossible to breach.

Unkept, unfocused, when you struggle to live.
Unintelligible, with nothing to give.

Unpromising, if you leave it alone,

Undependable, impossible to view,
Understand, the numbers and lines are a clue.

Hint

What am I?

• Is the last line for the numbers and lines of the puzzle, as in a hint? Or is it describing the thing you are talking about Commented Jun 29, 2018 at 23:25
• It's a hint, not describing what I am talking about. Commented Jun 30, 2018 at 5:58

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

Unknown

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.

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

• That is correct, good job! Commented Jul 2, 2018 at 15:18

I guess, it is ( a partial answer)

the horizon ( the view that one has on the imaginary location where the flat earth and the round sky meet)

For,

Unreachable, unattainable, always out of reach,

Obvious

Unstable, ever changing, impossible to breach.

One can never overcome/surpass it.

Unkept, unfocused, when you struggle to live.

It is there in all directions - not specific to a direction

Unintelligible, with nothing to give.

Not sure on this part.

Unpromising, if you leave it alone,

The horizon is usually blank/ empty!

Try going for it,(to seek an answer), to be in vain

Undependable, impossible to view,

True- as there is no such thing, not sure on impossible to see part

Understand, the numbers and lines are a clue.

What am I?

It is the horizon

• Not a bad guess, but it is incorrect. The last line especially is important in this puzzle. Commented Jun 29, 2018 at 16:43

Is it

The Future

Unreachable, unattainable, always out of reach,

We can never reach the future, it always escapes our grasp

Unstable, ever changing, impossible to breach.

The future is mutable

Unkept, unfocused, when you struggle to live.

Assuming you meant Unkept, not the more common Unkempt, the future neither makes nor keeps promises, no matter how hard you try, something will go wrong.

Unintelligible, with nothing to give.

We cannot make out what the future holds

Unpromising, if you leave it alone,

People who don't plan ahead are said to not have much promise

Those who look forward for answers will find nothing but hardship, for the future offers none.

Undependable, impossible to view,

Again, the future is hidden and fickle.

Understand, the numbers and lines are a clue.

We gain limited understanding of the future through our use of tools like calendars, which give us clear enumeration for the times to come.

• You're so close. In fact, when I was first thinking of the riddle, it was this word I was originally thinking of. However, I changed it the more I developed the riddle. Your last line, in particular, is wrong. If you follow it, you'll arrive at the answer. Commented Jun 29, 2018 at 18:49
• Since @Blake Steel’s answer is so close, I’ll hold back from answering, but my theory regarding the last line is that rot13(Yvarf naq ahzoref ersre gb n pnyraqne). Commented Jul 1, 2018 at 20:07
• Would I be wrong in saying the answer has a set of double and triple letters? (Triple non-consecutive ) Commented Jul 2, 2018 at 0:28
• @XenoDwarf, there is a triple not consecutive letter in this word. Commented Jul 2, 2018 at 14:31