Oh yeah, big time truth, lol. My mare got better as she got older, and improved a TON once I changed her diet. I took her off all feed products (sweet feed, feed pellets, etc) and put her on a forage based diet. She got all the quality grass hay she needed, a small scoop of chopped pre-bloom alfalfa hay, and half a small scoop of oats, along with a vitamin supplement designed for horses not getting any fortified grain. It's high in magnesium, which seems to help some mares as well.
Her attitude is 110% better! She still comes in season, but we can work through her moodiness with only a little extra effort. She's not as spooky as she used to be when in heat and in new areas. Her overall work ethic also improved. I HIGHLY recommend going to a more natural diet.