why am i suicidal and sad every single day, when I have no good reason to be?

There is a form of depression that is genetic and is caused by a biochemical imbalance within the brain that creates the depression.  This is called Major Depression.  It is different from endogenous or situational depression.

Please see your doctor for a prescription for an antidepressant medication and try it out for six or eight months.  If your depression is corrected by the medication, stay on it.

You can Google Major Depression to learn more.
Other people in your family have this form of depression, too.

There are different things that can cause depression. For example it can be caused by another condition, such as poor thyroid function. You would want to talk with your doctor.

The most important thing is knowing how to cope in a moment of crisis. It's very important to calm down. This will make you feel better and let you think clearly. 

A very easy way to calm down is just to breathe slowly. A doctor recommends inhale to a count of 4, exhale to 6. You'll feel better in 5 min or less.

Also, simple distractions can be very good. This video has suggestions. It also has a very good hotline. No one will bother you if you call.

https://www.youtube.com/watch?v=1yBXl81QPyY&t=7s

Take high doses of vitamin D and take some liposomal melatonin. 

Sadness can come over a person by losing someone or something that meant a lot to that person, but he or she didn't realize how much it meant until after it was gone.

You seem "clinically depressed" - stress can often cause this, and vice-versa.  If it lasts more than two weeks, see your doctor.

(It is normal to be "situationally depressed", such as if a loved one just died or you flunked a test.)