Ex-Miss USA Cheslie Kryst suffered from depression, says her mother April

April Simpkins says ... "I have never experienced such deep pain. I am forever changed," adding that the family confirmed Cheslie died by suicide last week in New York City.

She continued, "Although it is hard to believe, it is true. Cheslie had both a public and private life. In her private life, she suffered from high-level depression, which she hid from everyone - including me, her closest confidant - until shortly before her death."

April goes on to say that Cheslie's life, though short, was full of beautiful memories ... And, of course, she will be missed by her loved ones.

Her mother also said, "Cheslie - to the world, you were a sunshine wrapped in a smile. We talked, timed or texted all day, every day. You were more than a daughter - you were my very best friend. Talking to you was one of the best moments of my day. Your smile and laugh were contagious."

In the end, she said, "I love you, my little girl, with all my heart. I miss you desperately. I know that one day we will be together again. Until then, rest gently and in peace."

In lieu of flowers, her family is asking for donations to Dress for Success - an organization Cheslie holds in high regard. Her mother has also posted the number for the National Suicide Hotline ... encouraging anyone with suicidal thoughts to seek help.

As we reported ... Cheslie died Sunday by jumping from her apartment building in New York City. She lived to be only 30 years old.