I second the advice by KendraG.
Canadians are generally very easy keepers; it is unfortunate that this mare was allowed to get into such a condition. Definitely use an Ivermectin dewormer on her - other dewormers do not get all of the parasites. A visit from the vet to check her teeth, and identify any other issues, would be the first thing on my list.
1 cup of sweet feed wouldn't put weight on a piglet, let alone a horse. We used to only give it as a treat, not use it for any nutritional value.
I would also give her constant access to high-quality hay - don't limit the amount that she gets daily, just keep the feeder full and let her self-feed as much as she wants. She'll need that fibre to stay warm soon! Pasture forage has declining nutritional value at this time of year, as the plants are drawing their nutrients into their roots for next spring, so don't rely on that.
A complete feed is probably the best, and easiest, option for you. As the others have said, start small and slow and build up from there. It's going to be a long, drawn-out, EXPENSIVE process.
Frankly, when it comes to an animal you're not going to eat - like a horse - does it matter if the beet pulp is GMO or not? As long as it does the trick and helps her put some weight on.
Another high-fat feed you may want to consider is rice bran. I used to feed it to my senior citizen and it was excellent for keeping weight on. It's easier to feed on those cold days, too, as it's not a liquid.
You'll also want to give her access to salt and a good-quality mineral block. Check with UFA to see what they have that is horse-specific.
Good shelter - three sides and a roof - is going to be VITAL for this horse this winter. She will not be able to afford to burn any extra calories to stay warm, so good shelter is very, very important. Also ensure she has a constant supply of fresh, non-frozen water.
The market for Canadian horses is declining - it peaked probably 10 years ago, during and shortly after the push to get the Canadian declared as Canada's National Horse. Numbers are up, therefore, the demand is not as great. You can still sell them, but not for much more than the average quarter horse.