I agree with MerlinsHope.
Great Pyrennes are by nature watchdogs - livestock guardians. They will guard their "flock" or "pack".
Obviously you need to be seen as the leader, which it doesn't sound like the way things are now. If the Great Pyr is guarding YOU, he must feel you are weak and need guarding. You need to get WAY more confident and into a good leader-mode. You need to go to a class or have a lesson where your techniques can be observed - there are probably some things you are doing inadvertently that the dog is interpreting as weakness. A fresh pair of trained eyes will work wonders.
Also, never trust what people say when they give you a rescue dog. This is a hard lesson learned thru years of foster work.