AN: Written as a comment-fic.
He knew that by now he should have it under control, just like the control he was gaining over the intersect, but he had never been that good at controlling his emotions. He had always been a feeler as his sister would say, and he often took things to heart, after all look how long it took him to get over what had happened at Stanford. He can't stop his emotions, but he had learnt to hide them, even from his handlers, especially from his handlers.
Every time he sees Sarah he can't stop feeling jealous about her relationship with Bryce, CIA partners, proper spies, watching the others backs and lovers to boot. He thought about all the places that they must have been together and all the things they must have done together and he wants to glare, but he doesn't. He hides his feelings, but he just can't stop feeling them, instead he smiles at Sarah, a dopey expression on his face as he tries to battle through the jealousy.
When he sees Casey the emotions are just as strong, but instead of jealousy it's his anger that he can't stop feeling. Casey the super spy, NSA assassin, someone you don't want to cross. The man who threatened Chuck on almost a daily basis, hoping to get him to fall in line for the sake of national security despite the way the security agencies treat him. He can't stop his anger, but he does hide it, trying to learn everything he can from Casey instead in the hopes that it will help him later. His anger at the other man isn't really to do with how he treats him though, but how he had treated Bryce, how it had been his bullet that was the alleged fatal blow.
The main emotion that he can't stop feeling though is loneliness, he is the only person in the world to know how he is feeling. He can't talk about it as his very existence now has so many levels of security classifications, that knowing what he can talk about or not gives him headaches to work out. He could be less lonely if life hadn't thrown him so many curveballs, but the only people that he can really talk to are those prompting the emotions he can't stop, Sarah, Casey and Bryce.
If Bryce were there he wouldn't be so lonely, but the other man was out in the world working to track down members of Fulcrum in order to protect Chuck and the intersect. The thing that he really can't stop is missing Bryce.