My mother in law went through something like that.   Her dog (age 17) had to
be put to sleep and she kept asking for him over and over again.   Every
time we would explain it to her she would cry like she was hearing it for
the very first time.    Eventually she did begin to understand it, but it
took about a year or so and it nearly drove us crazy.   Sadly there was no
other way, and it had to be.

She was also deeply depressed and having delusions too.   Antidepressants
and antipsychotic medication helped her be in a happier more relaxed state
of mind.   Fortunately we were able to get her to take her medications, but
only after she came to live with us and we could supervise her routine.   I
have no idea how you could make your mom understand how much she needs her
medication, but if she doesn't start taking it, she will continue to be
unhappy, maybe for the rest of her life.

Also, we would not allow her to lay around in bed all day long and mope,
which is exactly what she did until we took her here to live with us.   She
would cry all day and not eat, dress, bathe or go out.   She lost a lot of
weight and looked (and smelled) just awful.   I think it is important that I
add that this was a woman who was an immaculate dresser and housekeeper,
always clean and neat and beautifully put together for all her life.
Alzheimers changed that drastically.

Once we got her here, we saw to it that she had regular showers and got up
out of bed and got dressed every single day.   We got her into a local
daycare program where they did all sorts of wonderful activities and crafts
and outings.   I think she had a happier and better quality of life when all
these things were in place than before when she would just lay around all
day and cry.

After she was on the right medication, hair styled, bathed, well dressed
again, she was a lot happier and even at times was a lot like her "old"
self.   If you saw her out with us, you would not think anything was at all
wrong with her (until much, much later).    Now that she is gone, we are
comforted in knowing that her last few years were good ones.

Think what you would want for yourself.   That is the best rule of thumb.
Would you enjoy your life more if you just laid in bed crying, or would you
prefer feeling a little better.... enough to function, at least, if a
medication could help that?

My husband became "the pill giver" around here, because his mother would
more readily listen to him.   Every night after dinner he would routinely
hand her a glass of water with her medication, and see to it that she took
each one.

Is there someone your mother might listen to more readily who could do that
for her too?