It is hard to grasp, or at least hard really to believe, the deep truth that fear is the opposite of love; because it is only true of Christian love of God, specifically, and of 'fear' that is the fear of the future ('angst') specifically.
But, as such, fear accounts for a great deal of the worst kind of misery in life; and love accounts for a special quality that is seen in the most faithful Christians - such as saints.
In my experience, fear can sabotage life when it is going well; and when there is some adversity, fear can make that (perhaps minor) adevrsity into something overwhelmingly significant, debilitating - fear can make adversity into depression.
In worldly terms there is no honest and satisfying answer to worry about the future; and no way in which that worry can be limited.
It is the nature of this mortal life that everything is uncertain, and all kinds of things might happen: and they really might happen. After all, terrible things have happening, and are happening to people all the time; plus, in a theoretical sense terrible things could happen to me (or those I love) at any time. Nobody can definitely reassure us that they will not.
External events may (by human motivations, or from natural disasters) overwhelm us. Our health is potentially fragile, this debility, pain or lesion may never get better, may get worse, may cripple or kill us...
Once fear gets hold; any apparently small life event (and there are always plenty) may become amplified to a grinding, persistent, increasing obsession; and life here and now be ruined by fear of what may happen; because nobody can say for sure that it cannot happen.
The only solution to this is love of God, and specifically love of the Christian God; who is the creator, who is our Father, and loves each person. The implication is that a God who is creator can order life and a God who is a loving Father can order life such that it will be for the ultimate benefit of each and every person.
My faith and trust in, love of, such a God is the only solution to free-floating fear; because it is rational. It is rational to assume that whatever happens to us in life will be for some good reason, and that our primary job is to to respond properly to whatever happens, to learn from whatever happens.
Yet this is not fatalism, because for Christians each of us is an 'agent', is a 'Son of God' with 'free will' - that is we all are a temporary-mini-god; so despite that our job is to respond and learn, we are not passive victims of life, we are not merely acted-upon by life.
Instead, our situation is one in which the situations of our own personal lives are a consequence, a product, of God's evolving creation interacting with our own genuine creativity; our capacity to have a personal motivation and to initiate new thoughts and actions.
God is the one source of creation that makes the arena, and each of us are point-sources of creativity within that arena.
What will happen is not predictable - but whatever does happen, God will be working (behind the scenes) to enable each of us to make the best of it; whether that best is sooner or later, during our mortal life or afterwards.
The confidence that this is true is faith-in, and love-of, God; and when faith and love are perfect - then trust in the future is perfect; and there is no fear.
The greatest saints therefore do not fear, and when we are (temporarily) in that saintly state of faith and love: neither do we.
But with most of us, we have not yet learned what the saints know; therefore our lives include serial, sometimes cumulative, challenges to our faith and love of God.
Something bad happens, or else we begin to be concerned that something bad might happen; and we respond with fear...
And it is by overcoming this fear with faith-in and love-of God - again and again, with different challenges - that we are learning how we need to be if we are to to make our personal choice in favour of the gift that Jesus Christ offers us: resurrected life in Heaven.