I think "it" is love that has faded.
His old or current lover still feels strongly for him, but he has inexplicably lost the passion he once felt for her.
He still cares about her, he wants to be there for her and I think that wishes he could love her romantically again. He doesn't know how to make his heart feel what his mind wants it to feel, and so he will just stop worrying about it and hope that things will work out in the end.
I think "it" is love that has faded. His old or current lover still feels strongly for him, but he has inexplicably lost the passion he once felt for her. He still cares about her, he wants to be there for her and I think that wishes he could love her romantically again. He doesn't know how to make his heart feel what his mind wants it to feel, and so he will just stop worrying about it and hope that things will work out in the end.