A few things before we start.
This fic was written for this kink meme prompt:
Pretty much what it says on the tin. Marie and the Cardinal were talking quite loudly just outside his door, after all. I want Louis' reaction to knowing he's a younger twin and that he's lost a brother and nephew before he could know them. Oh, and that his mother tried to usurp once again and was supported by the nobility because no one (save perhaps Treville, Richelieu, and Anne) believes he is or can be a good king.
Bonus for him telling Anne and she helping him through it.

The story takes places during episode 1x06 - The Exiles, and start during the conversation between Marie de Medici and the Cardinal, just after Louis has left the room.

The story is not beta'ed so any mistakes are mine and mine alone.

One last thing, I also posted this fic on AO3 under the same name.

Disclaimer: If I owned any of these characters I would not be writing fanfictions...

"Of eavesdropped conversations and loneliness"

Louis knew he was being childish when he slammed the doors and yelled, "It's so unfair". He knew that a king should not behave like this, but he could not help himself.
His mother's presence was taking its toll on him. She brought out that child who had felt his world crumble when his father died, that child that instead of being able to cry for his father death had to wear a crown that had felt so heavy. It did not help that the cardinal actually had the gall to forbid him to leave his own palace in front of her.
After he slammed the doors, he took a few steps inside the room as if he was trying to put some distance between himself and the people in the other room while he tried to calm himself enough to start acting like an adult again.
He took a few breaths, then went back to the door, perhaps to remind them that he is the king and not a wayward child, and reached it just in time to hear the cardinal say "…what big teeth you have grandmother".
He felt himself freeze.
Why was the cardinal calling his mother a grandmother? Louis did not have a child, he was certain of that. The fact that he had no heir was something that the nobles of his court would not let him forget so easily. The cardinal was probably referring to his sister's son, after all he and his sister are the only children of Louis XII and Marie de Medici, aren't they? Still why would they be talking about his nephew? Did his mother do something that could endanger his sister and his nephew? He certainly wouldn't put it past her.
He got lost in these thoughts and almost missed his mother saying that he was not only a weak man but also a bad king. It might sound strange but, frankly, Louis did not know which of the two was worse.
Truth to be told, he almost expected the words that followed. He had the feeling that one day or the other his mother would come back to try to take the throne from him again, and hearing the cardinal admit that he could imagine her strangle her own son to get said throne did not shock him as much as it should have. Rage coursed through his veins. How could she even think to do such thing? How could she even think that she would succeed? To think that for a moment he had actually been glad to have his mother back. He had actually dared to believe that she could love him, the child she had always thought of as a failure. He should have known that the only thing that mattered to her was the throne, just like she should have known that he would not give it up without a struggle. He was the king of France. He had a duty to his people. Still, she was foolish enough to expose her plan to the cardinal, so Louis couldn't bring himself to see her as a threat. He had no doubt that the cardinal would be able to come up with a plan to stop her before it was too late.
He heard her say "You know I could use a man of your talents." and almost laughed in disbelief while he waited for the cardinal to dismiss her proposal, to answer with that particular tone he reserves for those he regards as useless idiots. However, the silence lasted too long and doubt started taking hold of Louis. Could the cardinal really be considering it? Would he really betray Louis and side with Marie de Medici? Did the cardinal think he is a bad king too? Was there no one he could trust? Was he really as alone as he felt?
The rage he was feeling mixed with disbelief. The cardinal wouldn't join her. He simply would not. It was inconceivable. He would not abandon Louis, would he?
Breathing suddenly seemed such a hard thing to do, standing seemed an unimaginable feat to accomplish. He stepped away from the door stumbling. Louis somehow managed to reach the bed before practically collapsing on it, shock and betrayal washing over him.
He expected his mother betrayal, he knew on a subconscious level that it was coming but she was still his mother and it still hurt. The hurt of betrayal combined with the pain he felt when he realized that the cardinal had not actually refused his mother's offer were almost too much to bear.
If even one of his most trusted advisors could not bring himself to side with him, why should the people do it? Why should anyone else choose Louis instead of his mother and the unknown child she wanted to put on the throne?
The realization crashed on him and he felt so alone that tears started falling while he stared into the void, feeling to numb even to sob.

Anne, beautiful and sweet Anne, found him like that less than an hour later. Louis heard her come into the room and the first thought that crossed his mind was "Did she betray me too?" closely followed by "Did they send her to inform me that I am not the king anymore?"
He regretted immediately thinking of his wife as a possible traitor, especially when it took her only one look at him to get angry on his behalf because, somehow, she simply knew that his mother had broken his heart once again. So, when Anne asked him what happened he heard himself recounting the conversation between his mother and the cardinal word for word. She listened in silence, her attention completely focused on him, her hand holding his to offer comfort, to lend him the strength he needed and, once he fell silent, she hurried to reassure him, telling him that his mother knows nothing, that she would not succeed, that if the cardinal had dared to betray him he would pay for it.
It was in moment like these that he felt glad to have someone like Anne at his side.
Anne who stormed out the room mere moments later, fire burning in her eyes, while she promised to find captain Treville and his musketeers to oppose Marie.
Anne who came back a while later, a satisfied smile on her lips, dragging one of his musketeers in the room.
Anne who told him that his mother had left, her plans foiled by the cardinal, captain Treville and his musketeers.
Anne who indicated the musketeer and said, "This is Aramis. He has quite a story to tell you."
Anne, whose smile told him that maybe he was not so alone after all.