Hi Davina. Poor Dart. She looks like she's been through a tough battle. One thing to consider is her age. You said she's almost two? That's getting up there in betta years so any disease she contracts is going to affect her more than if she was a younger betta. My guess about her always being skinny is that she was stunted as a baby because of being a Petco baby. When they're that small, those high levels of ammonia in the cups do double damage. She's lucky to have made it. So many babies don't. You must have been taking such good care of her. :)
Because of she appears to be wasting away and because she looks like she has some excess slime coat going on (that would be the "fungus" stuff on her fins and body), she may have a parasitic infection, either internal or external or a combo of both. Some external protozoa can actually go internal once they burrow deep enough into the fish's body. This may be what happened.
After her Pimafix treatment is over, give her a few days in clean, dechlorinated water with no meds. Then I'd suggest trying AQ salt at a dose of 2 tsps per gallon as long as you don't have live plants or snails or shrimp. If you do, you'll need to either remove her to a treatment tank or remove the plants/snails/shrimp. I would also suggest increasing the heat to 86 F because this drastically speeds up the life cycle of many parasites. Do a partial change every few days, using a gravel vacuum to thoroughly clean the substrate if you can. If she hasn't shown any improvement after two weeks in the salt treatment, then we can try an antiparasite medication like Kordon Rid Ich or Rid Ich + or Mardel Quick Cure.
She sure looks like a little cutie so I hope she pulls through! Good luck with her. :)