You should read everything by anactoria. She does Dean angst and Dean/Cas like nobody's business. Here's her SPN fic on AO3.

You're too kind! <3

OP, if you're looking for epic later-seasons fic, there's Meet Me In Idaho (http://archiveofourown.org/works/1051339/chapters/2103613) (canon-divergent from 9.06)? It's Jossed now, but I remember enjoying it as it was being posted.

Also s9 era is These Forsaken Lands (http://archiveofourown.org/works/2398568/chapters/5302442), which picks up the aftermath of Cas's stint as God and IIRC is pretty angsty.

beenghosting (http://archiveofourown.org/users/beenghosting/pseuds/beenghosting) also gives good angst, and xylodemon (http://archiveofourown.org/users/xylodemon) writes a lot of episode codas that tend to be angsty and Destiel-flavoured.

I don't read much destiel, but I loved cucoo4cas's My Empire of Dirt (http://archiveofourown.org/works/1831612/chapters/3934216) (there's a long sequel, too). It's an all-human AU, VERY heavy on the angst (but the comfort is there, too), and lots of Dean pining after and resisting Castiel.

For later seasons:

Over the Hills and Far Away (http://archiveofourown.org/works/1407463/chapters/2952556) by roxymissrose: "Somewhere in the middle of season seven, this world careens towards the left. Dean looks at Sam and decides enough is enough. They need to settle down for a while, take a breath." (Sam/Dean)

The Start of Something (http://safiyabat.livejournal.com/6298.html) by safiyabat: "Castiel knew that Metatron killed Dean, but he expected his friend would make it to Heaven. When that doesn't happen he reluctantly joins forces with Sam to try to figure out what happened and how to fix it. He surprises himself by the pleasure he takes in working with Sam and soon comes to the realization that he loves Sam for his own sake. Convincing Sam of that, or that such a thing is even possible, may prove even more of a challenge than hunting down a super-demon and curing him." (Sam/Castiel)