If God exists, why do some life(s) never improves after years of darkness ?

Not sure why you would think that has anything to do with God.  God is not responsible for you - God had given you the tools - up to you to make something of them.  Some people are unwilling to make the effort to improve their lives, unwilling to be healthy and eat healthy food and live a healthy and active lifestyle.  They mope around eating junk food and sweets and on medication and thinking morbid and depressing thoughts instead.  Which sees rather a waste of life.  At least to me. 

That's one of the eternal questions many people ask, and many variants on it, such as,
if God is good, all-powerful, all-knowing and ever-present, how can He permit evil and suffering?

If you're a believer, it's best to ask the leader or teacher of your faith.
If you don't have one, but you do believe, you could try reading M. Scott-Peck's The Road Less Travelled,
or some of the ideas of the theologian Mathew Fox.

Scott-Peck is a Quaker.
Fox is a radical, mystical and very modern Catholic.

You could also try Eckhardt Tolle.

I guess that depends on what you believe about God.  I grew up as a Christian but I have a hard time believing in that god because the stories about it just don't add up.  I personally believe in reincarnation but I'm not out to convince anyone that what I believe is right and what they believe is wrong but the way my own personal beliefs answer this is that each soul has certain things it needs to learn during its time spent as a human being and every person is in a different place in their soul's progression so their life circumstances are there because maybe they need to learn to overcome them or maybe they just need to experience them as part of their karmic contract.  I hope that helps some.  I'm not real good at advice.  I'm better at listening to people instead of giving them advice but that's kind of harder to do on a Q&A site.

Christ was persecuted and died on the cross, we don't get excused from the sufferings of life, but what the bible says is hold on to God in all times.  This world is about birth, life and death, it comes with suffering, how you deal with that suffering is what makes a person, look at Job.  Satan said man will only have a relationship with God, so long as everything goes well, but if you take from him, he will turn away from God, "skin for skin" hence saying that mans' love of God is superficial.

This is the battle in the self as well, between our evil inclination that comes to us as a beguiling serpent tempting us to lie, to blame, to take temporary relief in things that will destroy us, or in telling us to turn away from God, that he does not exist, versus the good conscience, the fruits of the spirit  “But the fruit of the Spirit is love, joy, peace, forbearance, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law.” (Galatians 5:22-23) This post was edited by kjames at October 20, 2019 7:18 AM MDT

One need not necessarily be relevant to the other. Some find purpose or comfort in their god. Others find them in worldly pursuits.

In either case, you have to want it, be willing work toward it. But even that is no guarantee. Life is fickle, sometimes cruel, and challenges always arise. How we emerge from those challenges is what determines our character. Whether one believes a god is involved or not, it ultimately comes down to us and managing our circumstances. It is naive to assume life is fair, or easy, or orderly, or will eventually get better, or that someone is always looking out for them.
The most common fallacy I’ve ever heard a believer say is, “God wouldn’t give me anything I can’t handle“. Too many sad cases disprove this every day.

We can only do what we can and see where it goes. If there is a [benevolent] God, this is all He could expect. 

God exists when people bring light to its existence. Where there is no light, there is darkness.