My experience is similar to that of Phoenix I think, in that my dog started displaying guarding tendencies at a young age...not sure if it was quite at 12 weeks but certainly by 4 months we had had a couple of serious incidents that had escalated to multiple quick bites on hand and lower arm. Not sure if it's the same for you as your description sounds a bit different from our experience but I'd say any work that you can do that others have already suggested will stand you in good stead regardless of whether your dog is really guarding things or not.
I'll dissent from some of the other posts and say that whilst I know concepts of 'pack leadership' may be out of fashion I do believe that having firm, calm authority over your dog can help mitigate the extent of any guarding - BUT.... my experience is that you can not do this when the guarding is currently happening as the dog, like Phoenix says, will stands its ground and escalate the situation to more serious forms of aggression. Which is not what you want them to practice, as each time they do it is a learning curve for them that they can resort to this again in similar future situations.
Instead I think you have to work on this authority through training and the dog learning good general manners at times when it is more relaxed - when you do this you set up a more respectful attitude from the dog which might help mitigate any likely guarding. In my experience it's not the be all and end all (Ollie will still guard given the right circumstances like high value bones coupled with sleepiness) but it helps. I would also say work on the body handling - groom him regularly, get him used to body examinations where you look at paws, ears, teeth etc (give treat for positive associations) and keep an eye out for tendencies to guard locations like favourite spots on the sofa/ bed and train that out if it arises (these issues can tend to co-occur apparently and this was certainly the case with our dog).
If you're not happy with your trainer, change them. It doesn't mean that she is wrong in what she is advising necessarily, but you do need to have confidence in who you are working with.