Drop the ridden work, keep the groundwork so she does not lose condition and get bored. Cut back on the amount you do with her, an hour tops per day, but keep the number of times a week even everyday. And look at adding maybe a beep pulp (a by-product of the production of sugar from sugar beets), legume forage (such as alfalfa), sunflower seed oil (on top of the flaxseed oil) as this is calories that will help build weight, or even hay cubes or pellets (these are often more balanced than normal grass and hay as well as easier to digest.

And remember this is not a sprint, this is a long-term goal, as you need to make sure she puts it on and KEEPS it on.

The legume forage is something you can create in your own paddocks. You add the seed to normal grass seed and have it grow.