Where's my Momentum?!

So... recently I've been experiencing something I never thought I'd have to worry about with my warmblood mare. In the past six months or so, starting in the summer, she has become far more lazy. This is the mare infamous for her spectacular spooks and sensitive ways.

At first I thought it was just the summer heat or something. We all get a little sluggish when it's warm out... but it seems to have continued on into the winter. It's had a negative effect on our riding because try as I might, I can't seem to create enough "drive" and impulsion to be able to do anything useful. Now I'm almost happy when she does spook, because for maybe a minute or two afterwards she feels like the horse I knew... but eventually all of that energy peters back out and we're back to plodding around.

She'll be 17 years old soon... is she getting to the age that she's supposed to be slowing down like this? She does seem to have gotten much calmer with the onset of this lethargy... she feels more like a long-rein lesson pony and less like a dressage diva. We've talked to our instructor, the barn staff who work with her periodically, and even mentioned it to the vet when he came out for a routine check. No one seems to have an explanation for it beyond getting older.

Sometimes a change in food will help your horse pep back up. It doesn't take much but research the food you have verses some other food.

I knew a horse who was only fed a hand full twice a day(he was probably between 10 and 15), he was lazy as the day was long, but we changed his food, still only a hand full, and he had more energy and even had a better attitude towards being taken in from the field. :)