This was so intriguing to read! I liked the theme of calluses, and what they represent in dealing with pressure and pain, and the symbolism of shells being collected in this person's heart. I've never heard a verse worded like that, it's unique. It felt like a more poetic way of implying this person put a barriers around their heart, to protect them, to form a shield, to create this fortress to try and keep out any additional hurt. The way you wrote "not the beautiful kind" on a separate line filled me with an ache. The contrast of someone collecting sea shells as a hobby, as something that lifts their spirits, as a reminder of nature's beauty... versus someone collecting shells and building that barrier out of necessity, because they don't want others trying to find their way in and reach out to them, to see or acknowledge their suffering.
I could have read the last stanza wrong, but I believe it would make more sense as "until I saw how hard your arteries..." I feel like a word or two was switched around.
This also made me think of internal suffering, or the damage and chaos we may have in our body, our organs struggling, and the association of that with the things we hide from others. Or maybe our inability to accept the damage, whether caused by unknowns or something tangible.