Take into consideration your houses budget. I'm not sure if it's changed, but its possible that she doesn't have the money for it at the moment. Also, do you have the cage for a rat? The rat itself might not cost much at all, but the new foods it needs and so forth might.
Its also important to know your parents outlook on animals, if a rat is something that scares them or freaks them out, it would not be the best animal to have in the house. In that case, I would try for another animal. Im not sure if youd even want another guinea pig, but I doubt shed have a reason to say no to it if you still have the supplies.
I would save bits of lunch money and check out rat rescues. Or guinea pigs ones if you want. Point being, youd be saving an animals and it would be cheaper too. Try to convince your Mom that this is what you want. I know when I wanted something terribly from my parents Id save money for it. Thats the easiest way to get it.
Also, make it a point to her about how loosing your pet makes you feel. (emphasis on YOUR) I mean, DRILL IT IN. Sometimes parents just don't get how it feels because of a lack of communication. If she could truly see how hurt you are, im sure she wouldn't mind. In my opinion, I think the plastic rat was just rude. I would try to explain that to her calmly. Try using "I feel like..." more than "you did..." type of reasoning. Also, don't harsh on the "You promised" even if she did. It won't help her understand. It'll end up making you sound like a brat. However unfair it is.
Goodluck to you! Also if you need some rat rescues and so forth, contact me with your area, Ill do my best to find a reputable one.
