Let me start by letting you know that training collars are not effective for aggression situations. In fact, they can make it worse as the dog starts to associate the buzz or shock with the other dog and the aggression becomes worse.
The first thing that should be done is to rule out a medical cause for the sudden aggression. It is believed that about 10% of great Pyrenees have a thyroid condition which is one medical issue that can lead to sudden aggression. You can read about these here:
If there is no medical cause for the aggression, then it is strictly behavioral. I am leaning toward a medical issue since the attack was so long ago and shouldn’t have caused a change in behavior this long afterward. Though she may have become dominant as she matured into a full adult. Dogs are aggressive toward other dogs for a variety of reasons. It might be that they are fearful of other dogs and thus are aggressive before the other dog can be. In other cases, a dog is aggressive in order to dominate the other dogs and be the alpha member of the pack. Other causes could be that the dog feels they are the boss and as the boss they must protect you from threats (other dogs). She is a protective breed.
In addition, owners sometimes make the situation even worse by tensing up and worrying about what will happen. The dog senses the owner worry and feels that he is justified in his aggressive stance because you are obviously worried about the dog. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the other dog. For a dog like this, total control is necessary. This means not only physical control but on a mental level, you must be the boss. To accomplish this, you may want to have the dog wear a basket muzzle anytime she is not in your own house. This will not only prevent bites but also allow you to feel more at ease when walking her. If she is not spayed, have that done.
You will need to start obedience training back up with her. I don’t mean just giving commands to her throughout the day but structured training sessions each day. It doesn’t have to be a class situation though you will eventually need to finish the training in a class situation where there are other dogs present to test her training and teach her that she still has to obey then as well. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Obedience training serves various purposes. It helps a dog learn what humans expect of them when they state a command which leads to self confidence and less fear. Each time a dog obeys a command, even if it is for a treat, it makes them a little more submissive to that human in the future which helps with dominance aggression. And since it is the leader or boss who is responsible for protecting the pack, if the dog is made submissive with training, you are responsible for protecting him, so that can reduce aggression due to fear and dominance.
You will also want to keep a leash on her at all times initially to grab if she should disobey. I think you will be pleasantly surprised at how well your dog does with training. Dogs like knowing what is expected of them and they love the little paper thin slices of hotdogs that I use for treats while training. Give this a try and see how it works for you. The training collar I would save for long distance recall work. It is great for that purpose.
I personally am not a fan of dog parks because I see many formerly fantastic dogs changed after being attacked. They often become dog aggressive in order to fend off an attack and scare any potential aggressor away before they start something. It can be difficult to resocialize them but training can help. Instead of taking her to a dog park try taking her to an unused tennis court. It is usually fully enclosed and there are not a lot of dogs around. She can have some off leash run time without fear of other dogs being around.
You can also take her close to the park but not in the park so she isn’t right up against where other dogs are but I’d wait to do that until you have her trained a bit better. By better, I mean she will listen to commands immediately each and every time without hesitation and with distractions. You should be able to call her away from a fence with a dog and have her not only listen and come but not run back to the fence.
It will be helpful if you can find someone with a dog to help you once you have your dog listening to commands consistently. What you will do is have your dog on the leash. You will have your helper off in the distance. Your helper will gradually move their dog a bit closer to you preferably walking past your position in the distance. As long as your dog ignores them, you can give your dog praise and a treat. The second you see her fixate on the other dog or show any other sign of aggression (hair standing up, etc.) give your dog a correction by giving a short tug and a firm low toned "NO". It shouldn't take your dog long to realize you will not tolerate the aggression and that if she ignores the other dog, she gets treats. Once this happens you can repeat the training moving the other dog closer until she is no longer trying to lunge at other dogs. You will need to practice this when you and your dog are walking as well.
In addition, if the situation is not improving using the techniques on the previous website, you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.