RAB is right on with his worming schedule. I worm every 3 months rotating between ivermectin, Strongid, and Quest. Feb, May, Aug, and Nov.
Feeding sweet feed down here is very common, and having had horses in both Washington state and down here in Texas, I can attest to the differences in pasture quality between the 2 states. While a lush, fertilized coastal pasture down here is fine for providing nutrition to horses when its in full bloom, the majority of pastures don't provide as good a nutrition as those northern pastures do, and supplimenting with a bit of feed is justified. 10% sweet feed is not gonna hurt that horse any whether she is being worked or not. Since she is losing weight, she's obviously not getting the nutritional uptake she needs to maintain her body condition no matter what her activity level and warrants feed. I would recommend re-evaluating the amount that you are feeding her. As a starting point, for horses confined to a stall, I usually give 2 flakes of good quality coastal hay a.m. and p.m., and 1/2 a 3qt scoop sweet feed a.m. and p.m. for a 900-1000 lb horse, and adjust from there. Some will require more, and some less. Horses out on good pasture usually get less hay and feed, depending on the quality and amount of pasture available to them.
Since you also mentioned that she is new for you, were the previous owners feeding this mare? If so, would it be realistic to ask them what feed she was getting, how much she was getting and what her previous feeding schedule was? 10% sweet feed comes in alot of different qualities. For example, 10% Omolene brand of sweet feed is blended differently with more fat added than say Horse and Mule brand. Some do just fine on the lower quality, and some horses' metabolism require more added fat to maintain body and coat condition. She may be getting a different feed than what she was used to, and that will require some adjustment on either amount or quality on your part.
Losing weight may also be attributed to age, worms or teeth wear. Since you have addressed the worming concern, my next suggestion would be to have her teeth checked for sharp points. Is she dropping any feed when she eats? If so, you can (very carefully) slide your finger up into the side of her mouth and feel for sharp points on her molars, or a vet will be able to check for and file those rough edges down for you.
Best of luck with your new gal, and keep us posted on her progress!