I need help on my thoroughbred horse's nutrition! She's been losing wait and i don't Know what to do help!?

I have a 11 year old thoroughbred who I have been training to do barrels. She raced when she was younger and is very energetic.She has been getting skinnier and skinnier. But it doesn't seem to be that she is colliking. I try not to give her too hot of feed and here is what i give her:a flake of gr and a flake of alfalfa twice every day without any grain. What kind of grain would be the best for a performance horse that needs to gain weight?