What did the vet say happened exactly?
I'm a sorry for your loss. I'd recommend getting better educated about horses before getting another one. If that seems harsh then I am sorry for that too.
You said this horse went down on Sunday. You came to this board on Tuesday. The vet should have been there the second that horse could not get up not trying to call two days later. If I am misunderstanding and he was there on Sunday, you should have called him again when the horse went down.
If the horse was showing signs of weakness and illness it should never have been in the muddy area where it would have risked falling in the first place.
Find a local horse facility and volunteer there to get more experience with horses. They are certainly the best thing a person have but you have to really understand what it means to take care of them.