My philosophy on youngsters is to pretty much feed them all they want of hay, I just buy a big round bale and set it in the pasture.  7 horses go through one 1600 lb round bale of good grass hay/week in winter.  I also am very generous with the Omolene and Ultium.   Here's a copy of the Omolene 200 feeding chart to give you an idea of how you would feed the concentrates portion of her ration.  Remember, if your horse weighs appx 750 - 800 lbs (I'm just guessing from the pic you put up) and you want her to weigh 1,000 lbs, you would start out with only a little bit of Omolene until she got used to it, but then would increase until you were feeding the recommended amount for a horse weighing 1,000 lbs (the desired weight).  If I think the horse could use more calories, I give Ultium Competition in addition to the Omolene because it's high in fat (12.4% vs 6% in the Omolene). 
FEEDGUARD
 NUTRITION SYSTEM Purina
 Omolene #200
 Horse FeedProduct OverviewFeatures / BenefitsGuaranteed AnalysisFeeding DirectionsTestimonials
For performance, breeding and growing horses. . . Purina
 Omolene #200
 Horse Feed contains the Purina
 Amplify
 Nugget, a patented, high-fat, extruded particle that contains a proprietary blend of vegetable oils, flax seed and rice bran for an optimal fatty acid profile. Instead of using straight cane molasses which is high in sugar, Purina
 Omolene #200
 Horse Feed contains a unique blend of molasses and soy oil for a higher fat molasses that horse love to eat.
Feed Form: Mixed Grains and Extruded Nuggets
Recommended For: Performance, breeding, mares in foal, lactating mares and yearlings.
FEATURES BENEFITS 
Purina
 Amplify
 Nugget Patented, high fat, extruded particle that contains a proprietary blend of vegetable oils, flax seed and rice brand for an optimal fatty acid profile.
Vitamins and Minerals Provides 100% of the required nutrients for horses when fed as directed* so no additional supplements are necessary. *Based on Nutrient Requirements of Horses Sixth Revised Edition  2007 published by the National Research Council.  
Incredibly Palatable Purina
 Omolene #200
 Horse Feed is an incredibly palatable diet which is excellent for horses who are picky eaters.
Natural Whole Grains Nutrient and calorie dense blend of natural whole grains for lower feeding rates and less risk of digestive upset. Whole grains are a natural source of antioxidants, phytochemicals and fiber.
Less Bricking Blended molasses help to prevent bricking in the winter and drying out in the summer.
Healthier Hair Coat Contains balanced nutrition and added vegetable oil to promote a shiny coat and healthy skin.
Feed with Hay or Pasture Purina
 Omolene #200
 Horse Feed is designed to be fed with hay and/or pasture.
Feed Guard
 Nutrition System The FeedGuard
 Nutrition System is one of the industrys most innovative and exacting quality assurance programs. FeedGuard
 Nutrition System can help deliver the trust, quality and performance that horse professionals demand every day.
CRUDE PROTEIN MIN 14.00 % 
LYSINE MIN 0.70 % 
CRUDE FAT MAX 6.00 % 
CRUDE FIBER MIN 7.50 % 
CALCIUM (CA) MIN 0.90 % 
CALCIUM (CA) MAX 1.20 % 
PHOSPHORUS  MIN 0.50 % 
COPPER (CU)  MIN 55 PPM 
SELENIUM (SE) MIN 0.60 PPM 
ZINC (ZN) MIN 220 PPM 
VITAMIN A MIN 3000 IU/LB 
VITAMIN E MIN 130 IU/LB 
Omolene #200
 Performance horse feed 
    - WEIGHT OF HORSE IN POUNDS - 
LIFESTYLE 600 800 1,000 1,200 1,400 
Minimum Hay or Equivalent Pasture* 7.0 9.50 12.0 14.5 17.0 
Omolene #200
  Horse Feed (lbs/day) 
Light Work 3.25 4.25 5.25  6.25  7.25  
Moderate Work 4.25 5.75 7.25 8.50 10.00 
Heavy Work 5.50 7.25 9.00 10.75 12.50 
Very Heavy Work 8.00 10.75 13.50 16.00 18.75 
Early Gestation (first 250 days) 2.25 3.00 3.75 4.25 5.00 
Late Gestation (last 90 days) 3.25 4.25 5.50 6.50 7.50 
Lactation (when fed seperately) 7.25 9.50 11.75 14.00 16.50 
Breeding Stallion 4.00 5.00 6.25 7.50 8.75 
So to weigh 1000 lbs your horse needs a MINIMUM of 12 lbs hay or roughage and 5.75 lbs of Omolene 200/day.  Unless my horses are actively in performance or halter training I use the moderate work amount for horses that need to gain weight, light work to maintain. 
Here's the link to the Hennecke Body Scoring System, it explains exactly how to score your horse.   I prefer my horses a 6-7 score.   
http://www.habitatforhorses.org/rescues/bodyscoring.html
If you de-wormed your mare 3 months ago with Ivermectin, I'd use Safeguard or Panacur this time.  Next time I'd use one of the combo de-wormers (2 months from now, so if you de-worm on Dec 1, then do it again on Feb 1) and then Ivermectin again (2 months from then, April 1).  If you pick up your pasture, before I de-wormed again after the Ivermectin, I'd have the vet do a fecal count and advise which de-wormer (if any) he thought would be the most effective for you.  Since you only have 1 horse, if you pick up the pasture after her you should very shortly not need to de-worm as often as someone with several horses.  If you don't pick up your pasture, then it might take a bit longer to get there, but eventually I'd think you could cut back to de-worming twice/year if she remains your only horse. 
More ???, I'll be happy to try to help you.  I've been through and through the feeding and moving areas and switching hays and feeds and having my horses look great and then look like cr*p and then figuring out how to get them back to great again.   It can be very frustrating.