What breed is she hun? Looks like tb or tb x but cant tell on the pic. She is only young, you need a very high protien diet really to help her gain and keep weight and to be able to grow whilst in work. If i was you, i would turn her out for 6 weeks, no work, feed her a high protien diet and plenty of forage. You will notice a marked difference in 6 weeks beleive me. I have a 17.3h Tb who only farts and he drops 50lb. We are in the process of getting him back up to weight after him coming down very poorly just before i got him. He is not doing any work at all until he has got a good amount of weight on him.
Get your horse a showing mix, one that is high in protien, and high in oil, or add soya oil to her feed, this will help a lot.
Dont be too worried about it as some horses are weight shedders...it makes us feel terrible if they do it, no matter how much we feed, some dont seem to put weight on at all.
Also if you can, get blue chip feed balancer, this will help tremendously, lots of hay and if possible give her some haylage as this will help her gain weight as it is high in digestable protiens.
Basically; high protien mix. Blue chip if you can. sugar beet. chaff. and good quality hay, mix haylage in if you can. Just a suggestion hun.
Hope it works out for you. :)
A horse belongs in a stable not on a table!