I don't have experience with the hormonal implants. But the progesterone shot is an IM injection that cost about $15. It has to be given every 8-10 days. If it will work your mare will show improvement in a day or two. I have a boarder who uses mare magic on her mare and gelding and I do not notice a difference at all and my vet said it was worthless but the horses seem to think its very tasty.. It also looks like pot. I tried my mare on Smartcalm Ultra. She is in the middle of her second month and I didn't notice a difference. It smells like a vanilla shake but its alot of powder and she doesn't like it much.
I would try the progesterone, and if its a hormonal thing then you will know in a couple of days. Take her off of the sweet feed for a month and see if that helps too. If you don't then get your saddle checked and fitted and or get her checked by a chiropractor (I'm not really a big fan of chiropractors but it could be a last resort). I also agree with a previous poster that suggested more ground work. I think all horses benefit from more ground work.