He wants to leave, he does. But he can't. He knows what Merlin does when he 'works late', and he knows Merlin knows this. Yet they keep pretending; life goes on. It's a cycle, and Arthur's trapped. So helplessly trapped inside it, yearning to get out. But all cycles end at some point, Arthur used to tell himself. One day, I'll be enough for him again. But that day's not today. So he kept his head down and got on with it.
But lately it's been getting worse, not better. Merlin's out late every night; Arthur gets left alone.
Things used to be different. Merlin used to be his, and no one else's. They used to spend their evenings together doing god-knows-what; Merlin would smile that smile, the one he always kept just for Arthur, the one Arthur never gets to see anymore. They'd exchange looks that conveyed all their feelings to each other within seconds, and Merlin's eyes would be bright. They were happy, they were in love.
But now that love from Merlin has faded away into nothing, leaving Arthur empty and broken. Arthur spends his days working, now. He's taken up so many evening classes he can barely keep count of them all. This means that when he's not with Merlin, he doesn't have to think, he just has to work.
It catches up to him at night, though, when he's laying there awake. Merlin will be asleep next to him, breathtakingly beautiful, while Arthur will be wide awake. It's like Merlin's slowly leeching Arthur's sleep; making it fade just like he did with their love. Soon, Arthur won't be able to sleep at all, and Merlin will be gone.
These thoughts are the ones that rush through his head each and every night as he stares at the ceiling, silent tears trickling down his face.