Ok, I'm very annoyed now

Hello there []

Now, if you know my postings here, you will know how much this pains me to say, but this is often times a result of the dog seeing himself as the leader of the pack. [] The solution is easy: become a benevolent leader that sets boundaries and rules, that teaches her dog manners and basic obedience. You could start by getting yourself a booklet like Patricia McConnels "How to be the Leader of the Pack and have Your Dog Love you for it", or even much better, a full book like Jean Donaldson's "Culture Clash". Following their advise should put you back in control.

Good luck []

Thanks. I'll try some of that.

If this is an Aussie, they have very strong herding and guarding instincts.  The key is lots of training and mental stimulation so that the dog feels they have a job and you are the employer LOL.
Try clicker training.  These are dogs that soak up knowledge like a sponge and are good problem solvers.
Also, consider that your dog may not like the dog park environment any more.  Sometimes, when dogs reach adulthood, they are less than enthusiastic about the physical play they enjoyed as puppies and adolescents.  What you perceive as guarding could be that, but it could also be an effort to keep other dogs at bay, since they are not in your pack.  Herding dogs often feel uncomfortable about other dogs getting right up into their personal space.  It may be that they want a clear line of sight so they can keep their "livestock" in view.  Without observing your dog, I don't know, but it never hurts to engage the dog in obedience exercises.  If the problem persists, get a good trainer to help you out.

Sam already knows basic obedience, and I have been looking into agility... Is this an ok substitute?

Agility is a great confidence builder and will teach him to work away from you a bit, so I'd say try it.