I am going to go out on a limb here and speculate that a good portion of our "negative" emotions as human beings is based on one thing: fear of the unknown.
Let's deconstruct that a little...
Starting with the big one...fear of dying. Are we really afraid of the pain of dying, or the leaving people behind, or is it that we are afraid of the unknown? We don't really know what there is out there, after this, do we? Those with a strong belief in God and heaven will often say, "I am not afraid to die." That is because they believe that they know the unknown. They know what is waiting for them and it gives them peace.
The grief at losing a loved one, or the mere thought of it happening one day. The thought that someday we are going to have to deal with our parents getting old and passing on. It is excruciatingly painful to go through, or even think about going through. Why? Fear of the unknown. "I don't know how I will go on without this person." Unknown. "Why did they have to die?" Unknown. "Why them and not me?" Unknown.
A shy person will avoid many social situations that their less shy counterparts would jump at doing. Why? "What if there isn't anyone I know there?" Unknown. "What if I don't have fun?" Unknown. "What will it look like, how will I feel, who will be there?" Unknown.
Public speaking. Perhaps this one should have qualified as "the big one" as many people fear public speaking more than death. "What if I screw up? What if I forget my speech? What if they heckle me?" All unknown.
Taking a new course or going to school..."What if I fail?"
Asking someone on a date..."What will they say?"
Proposing marriage..."Will they say yes?"
Having a baby..."Will I be a good parent? What if I'm not?"
Starting an exercise program or a diet..."What if it doesn't work?"
Starting a new job..."Will they like me? My work? Will people be nice? Can I handle the hours/commute/workload?"
If a large majority of what we feel can be distilled down to one thing, then by conquering THAT, we should by default be able to conquer anything. By definition, we can't know the unknown, so what is the solution? The same as the one that the person who isn't afraid of dying makes. It is not in the knowing, at least not in the literal sense of the word. It is in the belief that we know. Beliefs are just thoughts that we think over and over until they are a part of our experience of life.
The next time you find yourself facing a crisis, reach deep down inside yourself and believe that you do know. Turn those questions into statements. "They will like me. It will work. I can do it." Say them enough times that they become belief. Be definitive. Replace doubt with certainty, for it isn't the knowing...it is the believing.