What is Miss Trotwood's old disappointment?

Betsey Trotwood is David Copperfield’s father’s aunt. This ”formidable person” is a sharp-tongued, rigid widow who thinks highly of her own opinions. Miss Betsey’s strength of character is shown when she pays her husband, who physically abused her, to leave and go to India, where he dies. After that, she re-takes her maiden name and calls herself single rather than widowed.
Miss Betsey is doubly disappointed. First, she disapproved of her nephew’s marriage without ever having met his bride, because she believed her to be too young and insipid (“a wax doll”). She became estranged from her nephew and only meets his wife after he dies and right before David was born. She arrives unannounced at their home one day and continually criticizes Mrs. Copperfield. Aunt Betsey decides that the baby will be a girl and declares that she will be her namesake. She expects to make the baby’s life easier than her own life has been.

I have a presentiment that it must be a girl . . . . From the moment of this girl’s birth, child, I intend to be her friend. I intend to be her godmother, and I beg you’ll call her Betsey Trotwood Copperfield. There must be no mistakes in life with THIS Betsey Trotwood.

A few hours later, when the doctor comes and tells her it’s a boy, she is certainly disappointed.

My aunt said never a word, but took her bonnet by the strings, in the manner of a sling, aimed a blow at Mr. Chillip’s head with it, put it on bent, walked out, and never came back. She vanished like a discontented fairy . . . .
https://www.gutenberg.org/files/766/766-h/766-h.htm

When David Copperfield's mother was pregnant with him, Betsey Trotwood was certain that she'd give birth to a little girl. In the days before pre-natal scans, all that people had to fall back on to guess the gender of an unborn child was their own feelings and presentiments. And Betsey has a presentiment that Clara will soon have a bouncing baby girl. She's so convinced of this that she vows to be the girl's friend and godmother. So when Clara subsequently gives birth to a bouncing baby boy instead, Betsey's understandably more than a tad disappointed. Indeed, she's so disappointed that she refuses to become David's godmother.
But Betsey's a kind old soul and she makes amends for her earlier impetuosity by treating David with great kindness when he shows up on her doorstep. He's just gone AWOL from the evil Mr. Murdstone's bottling factory and needs a place to stay. Taking care of David allows Betsey not just to get over her earlier disappointment at being deprived of a goddaughter, but her general disappointment with the entire male species.