Why did Sister Jeanne tell Sally's children that she has 'lost heaven'? Do you think she was being too hard on herself?

Why did Sister Jeanne tell Sally's children that she has "lost heaven"? Do you think she was being too hard on herself?

Sister Jeanne believes that she has "lost heaven" because she fed the invalid Mrs. Costello applesauce full of bits of peel and apple chunks that might easily choke her. Then, when Mrs. Costello does choke, Sister Jeanne doesn't try to help her and also prevents Sally from helping her. Mrs. Costello coughs and chokes, unaided, and after a few moments, dies of suffocation. Sister Jeanne has thus deliberately chosen not to do her duty as a nurse and as a Catholic nun by allowing the invalid to choke and die. She has acted in this way to enable Annie to be free to marry the newly widowed Mr. Costello (and thus avoid the mortal sin of sex with a married man), and also to prevent daughter Sally from taking her own steps to bring about Mrs. Costello's death (i.e., feeding the invalid a lump of alum in her tea that would probably choke her).

Sister Jeanne acted out of love and compassion for her beloved friends, Annie and Sally, but she knows that her action is a mortal sin. According to Sister Jeanne's strict beliefs, she has knowingly cast herself into a state of perdition and has lost forever her chance of heaven.

Was she too hard on herself? I would say that she was being too hard, but Sister Jeanne is judging herself by the rules she has always lived by. She is being consistent and unflinching, which to my mind takes an enormous amount of courage. She has given up her own hope of eternal life to save her beloved friends from mortal sin.

Borntoread does a fine job of discussing and answering this question.
I'd like to talk about Sister Jeanne's comment that she, "lost heaven a long time ago...Back when your mother was still a girl...Out of love I lost it."
the same love that Mrs. Tierney has earlier told her husband and Patrick Tierney's father, Michael - is a tonic not a cure. What Sister Jeanne did, she did from love - and it was a tonic, not a cure for sure. But she tells us, too, that truth is what God wants us to know in all things. She knew the truth and could admit to it - that she also says, "is how we will know him. So I do not truly believe she thought eternal life would be denied her. Also, later she tells Sally, "God is fair. He knows the truth." At the book's end we are told, "We marvel to think of it: how much went unspoken in those days....Well, the truth's out now." These stories reveal how much truly was not spoken in those days and in telling us this story, McDermott reveals that truth quite clearly. Sister Jeanne knew the truth, what she did, out of love - it was a tonic of sorts - and God knew the truth of it all - she would be saved!

It’s not clear to me what actually precipitated Mrs. Costello’s coughing fit and thus her death. Sally clearly planned to kill her with the alum. But what’s not clear is whether she actually drank enough of the poisoned tea to kill her. And Jeanne was going to feed her the applesauce, but I don’t think she actually ate any because she started coughing before the first bite. Between those two, I actually interpreted the alum as being what made her choke. And I therefore interpreted everything that Jeanne subsequently did as trying to protect Sally.

Borntoread says it all and says it very well. I also think Sister Jeanne judges herself much harder than God ever will. We know that sister Jeanne was extremely exhausted that day and did she simply forget to fix the applesauce appropriately? Did she not care? We also know that Sister Lucy raised her eyebrows when she saw the condition of the applesauce. Yes, Sally planned to end Mrs. Costello's life, but the alum had sunk to the bottom of the cup, Sally planned to stir it up and give her more, but never got the chance.
Sister Jeanne did nothing to help Mrs Costello' Her actions were out of love for Annie and Sally. I think her saving grace would have been to make it clear to Sally exactly what happened rather than let the girl go through life thinking she had killed the woman.

Sister Jeanne had a good heart and it’s unclear whether she purposely messed with the applesauce, but I think she will be judged less harshly than she judged herself

I think that Sister Jeanne told Sally's children that she "lost heaven" back when Sally was a girl not necessarily because of Mrs. Costello but rather because of her turning a blind eye to the affair between Annie and Mr. Costello. I believe that Sister Jeanne knew it had been going on for a long time before she confronted Annie about it.

This is a good question because it also makes the reader think there was something perhaps in Sister Jeanne's own life that made her "lose heaven" as well, besides the applesauce event with Mrs. Costello.

I think Sister Jean is carrying guilt more for the things she was thinking then for the outcome of her actions. Motivation is a strong precursor to guillt.