Hi & welcome,
despite 6 weeks ago doubling the amount of Purina 12 12 with adding a protein/fat weight gain supplement.
In addition to Loon's pertinent comments & questions, I will point out that horse's feed should generally be changed/added to gradually, and 6 weeks is often not long enough for much noticeable change. Also, as the 12 12 is a min/vit supp, why
did you double this? Did you learn that she needed more of all those nutrients, based on whatever her diet is? Along with Loon's analogy of a bottle of vitamins not giving enough calories, there's the consideration that too much of, or the wrong balance of certain nutrients can also cause more harm than good. So any nutritional supp should be carefully considered as part of the whole, well balanced diet. Which really goes for adding 'protein/weight gain supps' of whatever kind, to a large degree too.
So... perhaps with further info, it is just about what/how much she is fed, & we can advise you adequately without needing a vet. But if her diet IS OK, &/or she needs her teeth done, has gut damage, or something else is wrong, be it vet or otherwise, you're going to need to spend money I'm afraid. I absolutely - & I'm sure most of us do - sympathise with very... restrictive budgets, and am not in the least meaning to assume you are/will neglect her
because of it, but I'm with Loon, that if you can't afford to look after a horse adequately, then, sadly, it's time to find someone who can for her. Hopefully your financial restrictions are only short term/minor, so if it's a matter of vets etc, it won't come down to those questions anyway.