My mare was super twitchy/spooking at weird things and seemed just “tight” - magnesium made a big difference. It won’t do anything if they aren’t deficient as others have mentioned. I use SmartCalm Ultra as it is palatable, reasonably affordable, and seems to work!
I have had a mare that seemed to be more relaxed and laid back on raspberry leaves (much cheaper and the same thing as Mare Magic). They would be easy to try to see if they make a difference. However, the best way to determine if it really is a hormone issue is to try regumate. It’s not cheap ($2/day) but it isn’t as much of a hassle as I imagined (I just add it to her feed bags that I make up once a week). My horse had significant behavior issues that I thought were training problems or ulcers (until they continued after full ulcer treatment…and she didn’t seem like the ulcer-prone type of horse). I wouldn’t have described her as moody. One 14 day trial on regumate and I had a whole new horse who was back to her pleasant, cooperative self. She is now on regumate throughout the spring-fall although I may try switching her to raspberry leaves in a year or two (she is young and they can “grow out” of hormone issues).