Sick Betta - looks like she can't breath and is sinking...?

sounds like swim bladder disease, caused by constipation, you can help it if you can get it to eat blanched peeled peas.,

your bettas swim bladder is dameaged, so it is hard for it to swim, it needs to get to the top of the water to breath, especially if it is in an unfiltered tank, if it is filtered, it would add an air pump and small airstone ASAP,

in the meantime, take a cup full of tank water , and pour it back in from above the water surface so that bubbles go in the tanks, that will temporaroly add oxygen to the tank. then your betta wont have to get to the surface to breath as often, because there will be oxygen in the water, but you have to do this several times ever few hours, once the bubles stop, the oxygen will begine to run out again.

Have you changed her water recently, AND if you did, did you allow the water to sit for 24hrs ( or use a dechlorinating agent) to allow for any chlorine gas dissolved in the water to dissipate? Another compound called amine, is excreted by the fish. Too much amine or chlorine in the water will kill a fish, BUT chlorine and amine can mix together in the water to make chloramine, which is 100x deadlier than either of the other 2.

If you didn't use a dechlorinating agent and change the water again immediately.

i hope you have a filter and airstones in the tank and a heater set to 72 for them , and dont do water changes once a month let them get used to the water , that makes fish sick aswell , and go to the pet store aswell