I am sorry to hear about Merry and her problems. My name is Rebecca. I am a veterinarian and will do my best to help.
Some information about the other dog may be helpful. Male or female? How old? Are both Merry and the other dog spay/neutered?
I am sorry but I am not able to make calls for the website. I can continue with you here, or opt out and you can wait for an expert who can make calls.
I was afraid you were going to say they are both spayed females. I am very sorry to say, that in my experience and in the experience of all the behavior specialists and dog owners and breeders I have worked with over the last 30 years, the fighting between 2 female dogs is the worst. "Bitch on bitch" aggression just gets worse, as you have seen, and eventually ends with one of the dogs rehomed or setting up the household so that the two never can be together. Eventuallly one of the two is seriously hurt, or killed. I really wish I had a better answer for you.
You can try putting head collars like Halti or Gentle Leader on both dogs, with leashes attached, so you can pull them apart whenever aggression begins.
The problem is based in pack behavior, and in the wild if two females decide they can't live together, one of them leaves the pack and finds or starts a new pack. Living in a home, they can't leave, so the fighting escalates. We know certain triggers for fights that can be avoided: going through doorways, food, toys, who gets the good bed (usually an elevated position like furniture). Not allowing them to get onto furniture in the other's presence, not allowing them to be off leash together around doorways , feeding them apart and picking up food before letting them out can all be helpful, but then you still them fight over something insignificant such as a tissue on the floor.
This is one of the hardest behavior problems with dogs, and no amount of training will make them live together safely. You can keep both dogs, but it means crate training them, and one dog in the crate whenever the other is out, only be walked on leash, fed separately, trying the head collars.
Let me know what other questions I can answer.
You are welcome. Thanks for the good rating. Please let me know what other questions you have.