My grandmother has a spayed Westie bitch, and it wasn't totally indifferent for her. First, now her nose is very dry on one spot, the vet told us that it's because of not enough hormones. This vet knows another spayed bitch, which has to take hormones because of this, because her nose was completely dry, with cracks on it.
Secondly, she's less energetic and calmer (although still has her character, she would play with the other dogs, bark at the TV and so on
), but this means that she doesn't want to move (really, taking her out for a walk is a disaster - I always make at most half of the distance which I make with my own dog, because she just doesn't want to move!) and of course she's getting fat.
My grandmother also says, that after spaying she's less intelligent... but I don't know if it's true, or maybe the reason is that spaying changed her "lifestyle". But she seems to be less obedient, less attentive. I guess that it's similar to geldings. They are not so "busy" with everything, comparing to mares or stallions.
But I don't think that you need to breed her, to change her behaviour. My own dog (a Golden Retriever) is now 9 years old, and she never had puppies. She's still in very good condition and many people can't believe, that she's so old (I've read about Golden Retrievers, that they live no longer than 11 - 13 years). Once she had a beginning of "phantom pregnancy", and the vet told us that we should give her some more exercise and more interesting things to do. I started taking her for long walks daily, and also training her, and this stopped. Now she doesn't need long walks every day
But I know that Jack Russel Terriers are very energetic dogs, and they need a LOT of movement... for this reason they are very popular among horse people, because they are a good company for trail riding. Some time ago I met people who had a dog of this breed, and they were very proud of his "pseudo ribs" - muscles around his ribcage developed so much, that they looked like if his ribs were visible under his skin. He was really full of muscles and he could run with horses all day!
So for now you can take away her toys which she treats like her babies, and start taking her for long walks - maybe not very long to start with, because if she has phantom pregnancy, she wouldn't be in very good condition. But she should improve soon, and then you can make the distance longer. Maybe try some dog sport with her? Agility? Flyball?
When she's back to normal, you can give her toys back. But to my dog, one of her "babies" reminded her a puppy again, so I had to throw away this toy forever. I gave her new toys and she was fine.
If she has some fear issues (of course her aggression may be because of fear; maybe she's afraid of the males, if they are larger), you can make her bed in a place, where she can hide. Maybe a dog cage, which you can cover on the top, and on the walls, with a small entrance so the big dogs will not get there. It helped for our yorkie. We bought him a small tent, which was originally designed for a cat, I think
When he's playing with the Westie and it's getting too rough, he runs there to hide. Also when he did something wrong, or has a chew toy and doesn't want the other dogs to take it away from him. Sometimes he would also sleep in that tent. It makes him much calmer and more confident. It's important not to stress a dog in such place, even if you are angry or want to take her somewhere, she must know that everyone will leave her alone if she wants to go there.
I would also recommend you a book, "Click & Play Agility" by Angelica Steinker. It has tons of ideas for games, exercises, toys, etc. Great to read, and helps also with horses, even in such things like "how to motivate them", or "how to make them focus on you"
and there are some nice training plan ideas at the end of the book, which help you to organize everything.
Good luck