My little dog and big dogs

[DogEgg]

I have had my lovely rescue dog for a year now. She's a 4 year JRT/chihuahua mix (we and the vet think) and so is smaller than most other dogs we meet. After a year of work she's perfect in every way -obedient, clever, great recall and adores playing with other dogs. That is she adores other dogs up to the size of golden retriever, anything bigger and she is terrified and squeals, or barks madly seemingly going out of her way to make herself known to them. Certain breeds bring out the very worst in her - German Shepherds, Huskies and anything mastiff-like especially. She is as bad on and off the lead.

I've read up on this and tried classes (she's been perfect in the classes but her classmates have all been puppies and small dogs), doing a "look dog" thing from a distance and giving treats, relaxing and trying hard to send her positive messages when we see a big dog. She gets at least two good walks a day where we meet lots of other dogs, we play with the smaller dogs and I take us to a safe distance (before she reacts)with other dogs and then I distract her with treats or a ball before we move away. I don't want to push her closer as I worry it'll make her fear worse. She hasn't improved a bit in a year.

Mostly we avoid dogs I know she'll react to, but sometimes e.g. on a footpath, it's just not possible. I really worry that one day we'll find ourselves in trouble if I can't resolve this. I'd love her to be safe and confident in all situations and wonder if anyone can see what I'm missing, is it just a common small dog problem...or is this just something that perhaps arose before we got her and we just have to accept it's part of her? Any help would be gratefully received.