Resource guarding requires both actual training/behaviour modification sessions to address it AND management to prevent the dog experiencing the situations/contexts that trigger that need to guard something.
If you cannot, will not or have not done that, then it won't get any better, will almost certainly get worse and your dog is likely living with enormous anxiety much of the time.
Depending on what else you have attempted to address the RG with, it could take some time to fix, it usually IS fixable, its a job I quite like on the whole as the management is usually pretty simple and the fix very easy to understand and explain.
Sometimes however, a dog has gone too far, almost always when people have tried this and that and punished the dog for giving warning signs and tried to Cesar Millan their way out of it and do the ole 'she won't DARE growl at ME' type rubbish... those methods will over time produce a dangerous dog who is incurable :(
If you were my client I'd be asking you to speak to a vet who understands behaviour problems (not one who thinks its all to do with pack leaders and giving them a wallop), and in particular the link between pain and behaviour, and gut health and behaviour, and the use of anti anxiety drugs alongside behaviour modification.
If you can't/don't want to/won't do that, then I think euthanasia is the fairer option than rehoming - we have myriad dogs in kennels who do not have such severe bite histories, that are struggling for homes. We don't need more and the people who would take on dogs like this come under two headings..
1/ They think they know it all and they don't and shouldn't be allowed anywhere near.
2/ They actually DO know it all, but already have a houseful of dogs with issues and don't want any more thankyou very much.