It hit me like a ton of bricks! I let out an audible gasp when I got to that part; I really wasn't expecting it. And I think it works so well! As a reader I find I'm usually cool with things in an AU (characterization-wise) that I wouldn't be cool with from other types of fic, but this backstory could work outside the context of an AU as well, you know? It seems intrinsic to Matt, and you've weaved it in really elegantly. I wasn't expecting it at the start of the fic, but at this point, it's the logical outcome.
Like, at least some of Matt's formative years involved the thinking that "my body is a weapon therefore I am a weapon." So what happens when he's no longer capable of being a weapon? What's left of the self?

omg sorry I'll stop talking now; I'm just very invested in this fic and I think you're doing a brilliant job! :D