I have a roughly 18 month old red heeler that's been fixed. When she was young there was no problem with aggression or biting/nipping but in the last year it keeps getting worse, I've tried discipline through scolding, isolation, and holding her down to show dominance but none if that seems to work. Is it too late to correct this late blooming behavior or are there other strategies?

When a young dog nips you you should scream ouch as loud as you can. This will cause the dog to assocoiate bitng with getting the crap scared out of him from your screaming.

It's a HEELER! That's what they do. Give her a job or this situation will only get worse.

Research your breed. Learning and understanding why they do what they do will help you fix the issues.