I don't think you have to enter diets twice. I usually do it once and it looks like it doesn't change when it loads, but if I refresh the page or leave the horse's page and come back, it shows up how I updated it.
You don't have to enter diets twice. When you do it the first time and it refreshes, press F5 or Refresh and the diet will show up, or just continue training.
And for the forum, don't click Log In when you get on, click Check Private Messages and you only have to log in from there one time.