Older mare loosing her vibe?

My older mare (she isn't too old, she is 18), Rosie, seems to be slowing down a bit. Not speed wise, but energy wise. She only gets riden once or twice a week. Normaly she gets riden for around 2 hours at drill practice on Tuesdays. I only warm her up a little so she has more energy. On that day, in hopes of boosting her energy, she gets around a full folders can of soaked alfalfa cubes. She also gets half a can of sweet feed morning and night. Soon, like before the winter, I plan on getting her on some joint supplements since her joints are allways popping. She is an amazing horse and I don't want her to go to waste. She also appears to be loosing weight..I just found out that her teeth haven't been floated in quite a while, and this month the vet in coming out to work on my other mares teeth and I plan to have her look at Rosies teeth, too. Does any one know about maybe a senior supplement that boosts energy and helps with joints?