There was a heavy moment of thought that weighed on him for a moment, just barely causing his shoulders to stiffen. He hadn't had anyone back home to trust bar Kreacher. Between Gwen and Peter, he was slowly learning to let people a little closer. Peter knew his secret. One day, Gwen might as well.
Lupin was always steady and calm. A beacon amongst That Group. Regulus could see it even if his brother never noticed he saw it. Just like he was very keenly aware that they targeted Snape (who in Regulus' opinion had very much earned it, not that he'd ever be caught agreeing with his brother or Potter in public,) but never him.
There were things Lupin and the rest of them needed to know. Things Regulus didn't need to keep secret for his own safety any longer. Either he was stuck here forever or he was going back home to his immediate death. None of his boggarts could wrap their claws around him any longer.
He let out a heavy breath, turning back to Lupin and going to lean against the tree he'd jumped out of.
"Pettigrew," he said finally. "A couple years ago, I was in a room waiting for whatever Voldemort wished to do to me next. The door wasn't fully closed and I saw him change and knew immediately that Sirius would never in a million years believe me if I said he was there." Lupin might, but did it matter?
"And I realized that if it was something someone like Pettigrew could manage, then I absolutely could and there might be a way for me to escape...everything. Being quite good at wordless magic already and someone who was notorious for not speaking unless absolutely necessary, no one noticed when I was silent for a month or wandered off by myself." He'd make a comment about it all being so profoundly lonely, but what in his life wasn't?
"It was, until quite recently, the one thing I had that was just for me. A means to feel like I had some kind of control or a place where it felt like no one could hurt me." He swallowed a lump in his throat and looked up at Lupin. "Why would I trust the architect of so much of my pain, intentional or not, with that piece of me?"
He looked away again. "I know he'd just burst into uproarious laughter and declare that of course Regulus is a cat. Nothing in the world makes more sense! And nothing more would ever come of it because he just probably wouldn't care to think that deeply about it. Still, I..." He shook his head and decided to explain what his life was like in words he knew Lupin would understand without any further expectation. He took a deep breath and looked up at him. "I can't produce a patronus."
To admit out loud that there was something magical he couldn't do was the pinnacle of shame for him. "And yet, this part of me? It's precious and secret and mine and if I can't produce a patronus then at least I can do that and Sirius can't take that away from me too. No one can. Not even Voldemort."