How do I teach my puppy to be confident to be outside by herself?

I have a 5 month old newfoundland. She is an indoor dog but I build her a huge fenced in yard to play in with a lot of shade and a large dog house and everything she would need. The only problem is when we go outside and she realized I didn't follow she instantly runs back to the door and barks. I would never plan to leave her more than 20-30 minutes. I just want her to get some fresh air and exercise while I get my dinner started. Please help. I just want to make her less anxious.