As others have said, this horse need hay, not grains. Do you have a haynet? Really your horse needs hay at all times available. If you hang it in a small hole bag it wall last all day and not be wasted. If you stop buying the grains the money saved can buy the hay. They have dry bales of hay at tractor supply, and as others posters said the alfalfa pellets for added nutrition. Get a hanging scale and weight it out. My 13.2 hand pony eats 10 pounds a day of hay, plus about a pound of timothy pellets, my 15 hand quarter horse eats about 15 pounds of hay, about 2 pounds of pellets, and she gets to graze a couple hours a day. so, you should try at least 15 plus pounds a day, and since you are trying to get her to gain, let her have all she wants. If you can hand graze her (break her in slowly) grass is awesome for adding weight and giving good nutrition. I see green grass in the distance in the pic, can you take her there? but be careful to add grass slowly or she can colic (20 minutes first day...or less, and add 10-15 minutes a day)
I'd really concentrate on hay as a priority.