A woman has been hurt so many times that she shuts the world out, and the poem tries to explain why. Her feelings aren't gone. She has pushed them away to protect "the fresh and tender sores of her heart," a phrase that makes her pain feel raw and still healing. The long, plain line about getting hurt "too many times to let just anybody in" reads like a simple reason, almost an excuse she doesn't owe anyone.
The speaker seems sympathetic and a little defensive on her behalf. Other people look at her distance and call her "heartless," but the poem pushes back on that. The last line says she is "hidden and still beating more than anybody would know," which turns the insult around: she feels deeply, maybe more than most people. The mood is quiet and guarded, sad at first, then gently hopeful and protective, as if the poem is asking readers to look past someone's cold surface.