Why is Miss Brill a lonely old woman in Mansfield's "Miss Brill"?

First, Miss Brill doesn't know she is a lonely old woman
until the Romeo and Juliet couple in the park tell her she is. Her life has been rather
pleasant up until this encounter with Romeo and Juliet; it has been rather like her
Sunday honeycake that generates great anticipation and "a dashing way" to her
life:

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Sometimes there was an almond in her slice,
sometimes not. It made a great difference. If there was an almond it was like carrying
home a tiny present ... She hurried on the almond Sundays and struck the match for the
kettle in quite a dashing
way.

Second, we are told very
little about any of her circumstances but we do
know:

1. she feels pleasantly happy at all times,
   which is confirmed by her dedicated attention to her beloved
   fox.


2. she has two costly and elegant possessions that we
   know about: the fox fur necklet and the Eider duck down eiderdown comforter, coveted by
   luxury lovers.


3. her room is a small one as the red
   eiderdown is before her as soon as she walks
   in:

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she ... climbed the stairs, went into the little
dark room–her room like a cupboard–and sat down on the red
eiderdown.

We can make
inferences from these random bits of information. We can infer that she once had a more
elegant life and that she had some level of social prestige. We can infer this because
of her fox and red eiderdown, which are each costly and a suitable part of a less
constrain lifestyle.

These puzzle pieces don't provide much
of a picture from which to answer your question but the cumulated inference is that
once, she lived differently until some unknown circumstance occured (perhaps the death
of her parents and loss of wealth or the death or loss of her fiance ...) that reduced
her situation to near poverty, though she took it bravely and kept up her spirit and
good cheer.

She may have cherished a secret hope that at
sometime, things might change and she might regain her former position. With such a
secretly cherished hope, time passes without being noticed and the remarks of Romeo and
Juliet might have shattered the mental images of her life that she had frozen in time.
So she may have become (not been) a lonely old woman that day
because she recognized the reality of her life and future or because her secret hope was
shattered.