Former Madeira School headmistress Jean Harris, who recently called prison "a terrible, terrible way to live" with "the only thing that sustains me . . . the hope of getting out," today lost another appeal for her freedom.

Harris' conviction in the shooting of her lover, Scarsdale Diet doctor Herman Tarnower, was upheld unanimously by the seven-member Court of Appeals, the highest court in the state.

"Jean Harris was convicted following a jury trial of murder in the second degree and criminal possession in the second and third degree," wrote Judge Dominick Gabrielli. " . . . . Our resolution of [the] issues leads to the conclusion that reversible error was not committed."

Harris, now 59, is serving a sentence of 15 years to life in prison, mandatory for a second-degree murder conviction.

Convicted of killing Tarnower in a jealous rage, Harris became a household name after her five-month trial. Books and movies appeared.

Harris steadfastly proclaimed her innocence. She had shot Tarnower, her defense team argued, in a "tragic accident" -- a suicide attempt gone awry. At her sentencing in March, 1981, wearing a designer suit, she made the claim herself.

"I want to say that I did not murder Dr. Tarnower," she said. "I loved him and I never wished him ill . . . I am innocent as I stand here . . . for you to arrange my life so that I will be in a cage for the rest of it and that every time I go walk outside I will have irons around my wrists is not justice. It is a travesty of justice."

Ineligible for parole for 15 years, Harris has twice unsuccessfully appealed her case. Last December, the Appellate Division of the state Supreme Court upheld her conviction, saying that while Harris "did not receive a perfect trial, she received an eminently fair one."

In her most recent appeal, defense attorneys Herald Price Fahringer and Paul Cambria argued that pretrial hearings prejudiced a fair trial. They also said the testimony of a police officer -- who overheard a confession by Harris -- should not have been allowed in evidence.

"Oh, My God, I think I've killed Hy," Harris had reportedly said.

The court today said there was no evidence that pretrial publicity affected the trial. The justices also said that Harris' rights had not been violated when the conversation was overheard: "A statement made by a defendant who has invoked the right of counsel may be admissible at trial if those statements were made spontaneously."

Harris, through prison officials at the Bedford Hills prison outside New York City, declined to discuss the court's decision.

New York State Department of Corrections spokesman Lou Ganim said, however, that Harris was "doing very well" in the prison system, working as a teacher's aide, and with a "parenting" program for imprisoned mothers.

Harris, in an interview and article in McCall's magazine this past September, painted a much bleaker picture. Writing about prison life, she said "from breakfast to bedtime I have to wait for 60 doors to be unlocked." She attacked the prison system eloquently and fiercely. "America has done a splendid job of redesigning its zoos and developing a whole new kind of enlightenment about the habits and needs of animals. It's time we did the same thing for humans."

Defense attorneys have said they will once again appeal the case. CAPTION: Picture, Harris after verdict: paints bleak picture of prison life., Copyright (c) 1981, Gannett Westchester newspapers