Horse Veterinary Questions? Ask an Equine Vet.
I am sorry your horse is struggling. It is no doubt a worry and certainly given that it is a new development, I would get her checked by your vet.
There are a few things which could cause such behaviour. Cardiovascular disease could be a cause so your vet should thoroughly listen to her heart and lungs and measure the recovery rate of the heart following exercise. A full clinical examination will also reveal if there is any infection present that may be affecting metabolism and performance.
Sweating can also be seen in exertional rhabdomyolysis and this can be serious so your vet can take some blood and look for signs of muscle cell breakdown. It is important that after such sweating she is rehydrated and an electrolyte solution could be given to replace vital salts that may have been lost through sweating.
Apologies for the misinterpretation.
Having worked over in New Zealand with racehorses, we would often give a pre-race cocktail of various vitamins and minerals to help with hydration and muscle work, I always added in Kynoselen. I cannot comment on its current legal classification regarding racing.
If anxiety is a definite issue, then there are some products which are general calming agents for anxious horses and I am sure they wouldn't cause any issues on testing, products containing L-tryptophan, passiflora incanata, L-theanine. Again, you would need to check these individual ingredients in case they were banned/controlled substances for competition horses.