The loner learned his lesson. He will no longer declare I Work Alone; he has made friends and learned to trust them.
So why is he sneaking about with secrets? Aesop Amnesia?
Nah, just bad habits. One of his True Companions just has to remind him of the Power of Friendship and the Power of Trust. Let it be known that his mistrust is hurtful. Distinguishable from Aesop Amnesia in that they do not have to learn the lesson over again; the reminder often suffices, for an explanation of secrecy if not an actual explanation.
A form of Character Development more realistic than simple Epiphany Therapy at that.
Sometimes needed when the character learns new things and tries to keep them secret. Sometimes can be dangerous, if there are good reasons for secrecy.
A lot of this between Kotetsu and Barnaby from Tiger & Bunny; even after they get to the point where they start to trust one another, both have their reasons (with Kotetsu it's his fear of "becoming a burden" on someone while in Barnaby's case - since Kotetsu is his first and only friend and confidant - the habit of twenty years) for not sharing certain things with one another.
Gleefully deconstructed in Attack on Titan, after Eren joins the Scouting Legion and finally makes friends with people who want to fight the Titans as much as he does, his first mission with them involves trusting Levi's squad to escort him safely through a Titan-infest territory—as opposed to going on a one-man rampage in his Titan form. The whole gig ends with the entire squad being wiped out before Eren realizes that The Power Of Trust is worthless against the Titans and goes on a one-man rampage in his Titan form. Which ends with him almost being captured by the enemy. Yeah, it's that sort of manga.
In the Age of Apocalypse, Rogue frequently reminded Magneto that he had promised not to keep secrets from her.
Predator 2. After Danny is killed by the title creature, Lieutenant Harrigan wants to take it down by himself.
Leona: Mike... Mike, goddamn it. This ain't your personal little war, you know. I loved Danny too.
Jerry Lambert: And you told me, lieutenant, "the only way you survive down here is because you're a team." "The door swings both ways," remember?
In His Last Command, Kolea finds Rawne and the others from the Geroen team meeting in secret, and blasts them for forgetting their Fire Forged Friendship and not trusting their commander.
In Blood Pact, when Curth stumbles on the secret transmitter, she wonders why they kept it so secret. Hark explains that he was trying to keep it secret as possible, so they wouldn't be hurt by knowing.
In Cold Days, Thomas rages at Harry that Harry was obviously dead, because if he was alive, he would have come to Thomas for help.
This trope is a major part of Dresden's Character Development. At the beginning of the series he is a loner with only one or two acquaintances, and no real friends.note Arguably, he was friends with Michael already, but since that character isn't even hinted at till the third book, he can't really be counted As the series progresses, he gathers an ever larger group of companions, friends, and blood brothers. Learning that it's okay to depend on other people is a very hard learned lesson for Harry.
In Dorothy L. Sayers' Busman's Honeymoon, Harriet tells Bunter to tell Lord Peter Wimsey that she is sitting up the night of the execution. In due course, he shows up, apologetic for having — he admits — forgotten.
Happens repeatedly in the Mistborn series, as the protagonist Vin is extremely paranoid thanks to her upbringing. Then it gets turned around a bit in the last book; Vin can't talk about her plans or else Ruin will hear, so Elend has to just trust that she'll make the right choices.
Live Action TV
Mac Taylor from CSI: NY suffers from this frequently. After the death of Angell,when he is caught up with the solving the case,Stella actually tells him that he isn't alone. It later became a big part of his recovery from being shot during the season 8 finale. Mac struggled with a speech aphasia condition as a result, and although his friends knew something was wrong, he wouldn't open up to them. He told the one friend he did confide in that he felt it was his problem to get through and he didn't feel the need to bring the others into it. Jo,Stella's replacement, confronted him but he rebuffed her. He later apologized, but still insisted she not keep pushing. His girlfriend, Christine, called him on it and was pushed away as well, before Mac finally apologized and opened up to her when he realized she was ready to leave him. Christine again reminded him that he wasn't alone between her and his friends. Whether he listens remains to be seen. This time, it was probably a combo of typical male ego/reluctance to appear weak and concern that if too many people found out, it might endanger his career.
Lindsay has a mild form regarding the guy who killed her friends. Even now that she's married to Danny, she still got reluctant to let him in and let him support her when the guy's execution date came up.
Michael Westen of Burn Notice also sometimes needs this reminder, perhaps the most clear-cut case being in "Enemies Closer":
Gossip Girl: In three consecutive episodes from the third season, Chuck draws away from Blair while dealing with a serious situation, then after a gentle prod from her he remembers she's there and opens up to her.
NCIS: Subverted. Gibbs isn't willing to draw his team into a case where he's breaking the rules to protect his old CO. Tony and Kate are hurt and assume that he doesn't trust them, but Ducky explains that while Gibbs is just protecting them. Then they play it straight by reminding Gibbs that they're there for him.
In a Schlock Mercenary arc, after the Toughs discover that their memories have been tampered with, Petey offers to help set them straight. A leery Tagon asks Sergeant Schlock (whose unique neural physiology means he's the only one with an accurate picture of what's been going on) if he remembers anything that might help him make a decision.
After the first season, there's a few times Robin had to be reminded that all the Titans are his friends and want to help him.
There are times when this trope applies to Raven as well.
In Ben 10: Alien Force, Kevin Levin can't seem to wrap his mind around the fact that Ben and Gwen trust him now that's working with them to stop an Alien Invasion, if his stealing their grandfather's RV to trade for an important message from said grandparent is anything to judge by.
Lion-O from ThunderCats (2011) had this problem for a while into the show, understandable considering how many times he willingly offered up a plan and had it shot down by someone like Tygra or Panthro.