Sure that’s not Depo? That has to be timed with the cycle and know several who give it at 10 day intervals.
For OP, know it’s costly but if she’s showing through this time of year, upping the dose is about the only answer whether injected or oral. Probably only need to do that a few months of the year as indicated by her behavior and it’s easy to adjust the oral dose, you are stuck with the injectable.
Don’t use marbles. Inserting a foreign object into the uterus is a hold over from days of yore, like back to chariot racing, when it was practiced mainly as a way prevent pregnancy. They can expell the marble and it’s more likely to cause infection then manage hormone levels to control behavior during estrus. Despite reading the miracle of marbles on here all the time, never, ever knew anybody who actually used them regularly. Oh, some cited cousins sister in law, friends neighbors uncle or great grandpa out in Wyoming in the 1920s, but nobody actually used them. A few admitted trying it but it didn’t work, they either were found on the ground within a few days or they stayed in and did absolutely nothing to change undesirable behavior.
Had a few mares, think the first one I bought in 1969. All but one were not bothered by hormonal behavior other then an occasional pissy day, which was easily dealt with by enforcing proper training and handling.
The last mare was bothered. Big time, Was older, like 10, when I got her, was told her mother was the same way and she was bad, bad, bad. No ground manners either, even when not cycling. Good thing she was the last one and not the first or I would have been in over my head with her. Fixed what I could, converted her to my standard of broke, including ground manners. Tried everything, except marbles. The only supplements that sort of worked worked had sedating substances and it was a rated show Hunter so that was out. Went to Regumate, needed to go to max dosage some months. No problems in anestrus so she came off the RM October to March.
She was, however, a hell of a good horse down the lines and not only saved my ass but made it look good. Sound, easy keeper, good traveler. All heart and try…except for three days six months out of the year. Sometimes you do need to negotiate.