I'm definitely not the first to rec this work, but it deserves it all. I had it open in a tab since the posting date probably, but couldn't get around to reading it, probably having suspected that it's not going to be so easy.
Tug of War (33268 words) by herbeautifullie Summary: Arthur really should have listened when Morgana told him that having a child at someone else's demand was a bad idea because, though he'll never say as much aloud, she was correct. It started off with Merlin spilling coffee (which was far too common, really); there was a bit of paint and a baby in the middle and then, in the end, there was a break-up... Except, that wasn't the end at all. Arthur wouldn't let it be. [Thoughts]
Thoughts: Let me say beforehand that it does have a happy ending, because a lot of people only agree to handle angst on that condition. That taken care of, thoughts.
This story is... a harsh reality check. The author takes Arthur and Merlin, with all their flaws, not forgiving or forgetting a single one, places them in a modern setting, and doesn't let them slip into the 'pretty lie' territory once. Yes, they love each other, but sometimes it's not enough, as it so often isn't in life. No, Arthur isn't a heartless bastard, but he is hung up on his father's will when he shouldn't be. No, Merlin isn't a pushover, except in small things, and you kind of hate him for it, but even those are true to form. Uther isn't a monster, he's just very much Uther. Despite her prominent role in the summary, Morgana isn't actually that big a part of the story at all. The story itself is a life-long journey. It has a classic rom-com beginning, but after that it's all brutal realism all the time. It's very intense. It's not an easy good-night tale. However, I dare say that it resonates with many of us on a great many levels, and, while fandom is all about escapism for me, stories like these are good for the soul sometimes.
I'm not sure when/if I'll be rereading it for the sheer quality or this particular brand of angst. But I am sure that I'm unlikely to forget it.