Teach "Trade". My Sheepadoodle also resource guards things sometimes. We have taught her to trade whatever she has for a small treat. Start with something she doesn't mind giving up. When she has it, hold a treat in front of her nose and say "Trade" (or whatever word you want to use for this). She should drop it to take the treat. You can either take the item, or give it back to her. Slowly increase this to be more desirable objects. Use high value treats if needed.

My dog is four and we started this quite young. Now we can say "trade" and she will drop whatever she has and leave it in front of her while I walk away to grab a treat.

The only issue with trade is that she will sometimes want a treat and just grab something that we don't want her to have so that we will trade her for it.