In Notes on "Essentials on Faith Alone", Shinran clarifies the meaning of "diamond-like shinjin":

When sentient beings realize this shinjin, they attain the equal of perfect enlightenment and will ultimately attain the supreme enlightenment, being of the same stage as Maitreya, the future Buddha. That is, they become established in the stage of the truly settled. Hence shinjin is like a diamond, never breaking, or degenerating, or becoming fragmented; thus, we speak of "diamondlike shinjin."

When I initially read that shinjin should be diamondlike, I thought it meant that we should always feel something, that shinjin should express itself in a certain way, for example always rejoicing, feeling at peace, feeling joy. But we know this isn't the case because Shinran himself lamented his lack of joy while still being sure of his shinjin. However from this passage we see that diamondlike shinjin actually refers to entering the stage of truly settled meaning that there's nothing that we or anyone else can do to hinder it. This includes not rejoicing in it, or perhaps even forgetting it. So we shouldn't worry about the expression of our shinjin in our lives, diamondlike refers to how no matter the expression of it, we can't move away from it.