Hi, kismet,
it's probably not hopeless - a lot of betas have been in similar circumstances and come through with proper treatment.
Speaking as a non-expert: have you tried feeding tiny bite-sized bits of pea?
A lot of people feed peas weekly, as well as in emergencies - although regular pea-feeds tend to prevent the emergency part.
How long were/are the epsom salts (I believe salts actually have to be removed with water changes - no need to remove charcoal) left in the tank?
They help the fish absorb moisture, and while long-term use is not good, they can apparently be used for up to about a month when necessary for treatment.
Some people soak pellets prior to feeding, as they can swell up in the stomach and create such problems.
If she's prone to constipation, might want to consider doing that in future.
Although fish aren't made to digest grain, it's typically in commercial fish food sold in LFSs - it's surprising there aren't even more problems than there are.
Assuming she has a heater, what temperature is she at normally and right now?
Has she pooped at all?
I'm hoping somebody superhot at betta diagnosis and advice like LauraFrog or Wilder will show up soon - if they don't, you could try also posting in the emergency section; it really does sound like constipation.
In the interrim, you might try posting a few details on your tank, thereby helping identify potential issues.
They're such a worry, aren't they?
Edit; changed one part of a sentence, and not the rest to match.