This series takes place after Age of Ultron and focuses on Bucky dealing with Steve's loss. It's also got a lot of Natasha in it and, so far, I really like the way the author writes their POVS - http://archiveofourown.org/series/462613

And a little shameless self promotion - I wrote a really short one shot about Bucky having doubts right before his first mission as Cap http://archiveofourown.org/works/4132114