The pain, the physical ache, the ever-present cold that seeped into his heart, into his very being, had certainly been part of why he had chosen to take the voyage, to leave behind everything he had ever known for a world entirely unfamiliar, entirely new.
He had shared some of it with Sam, though as little as possible. Sam had other things to occupy his mind, other things he needed now to be concerned with. Worries for his master who could not be helped would not have done any good to Sam's heart, Frodo knew. So, he had remained quiet, keeping his aches, his pains, his need for relief to himself as much as was possible.
And there was more too. For the wound from Weathertop never did fully heal, and there were times that Frodo felt he had never truly come back from the fading that had begun. He would wake from a nightmare, and welcome the darkness, for it seemed gentler and less harsh than the bright light of the sun. He would wander from his bed, cold and shivering, yet desiring no warmth. He felt that he himself had ceased to be, that he was no longer real, that he truly had become nothing more than a wraith, a shadow.
But more than the pain, more than the nightmares, more even than the feeling of being between the worlds of the living and the dead, it was the ever-present hunger, the always nagging desire for that which had so wounded him to begin with. He despised that he still wanted it, hated that his need for it dimmed the joy, the beauty, the goodness of everything around him.
He would dream of it often. He could see it, just out of his reach, more shining, more perfect, more beautiful than ever. His eyes would grow wide as he reached for it, his fingers just barely brushing that gold that nothing could mar, wishing only to hold it, to possess it again. Sometimes, he would succeed. He would take hold of it once more, feeling the weight of it in his hand, feeling a sense of victory, of power, of delight at having it back on his finger again. But then he would wake, the healed scars on his neck now hurting like they were freshly made wounds, the spot on his chest where that accursed thing had so long rested would burn with an intense heat that was somehow cold at the same time.
And the emptiness he felt, the despair that it was really gone, that he could never see, never touch, never hold it again, would nearly consume him.
That, more than anything else, is why he knew that he must go. In Middle Earth, he would always long for it, long for what he could not have, desire what he knew would have been, what had been his own destruction.
It was gone now, and the Shire had been saved. But Frodo was now so twisted, his mind so dark a place, that he hardly recognized himself. He knew it was no way to live, forever longing for that which could not be.
So yes, he would sail. And when Sam asked why, Frodo would tell him, but not the true reason. No, that bitter truth would be better left for the day when Sam would join him in the Undying Lands, when Frodo had been healed, cleansed, mended by the gentle tending of the Valar that Gandalf assured him he would receive.
But for now, if he ever wanted to escape the cares of Middle Earth, the hurts he had received, if Frodo ever wanted to feel truly alive and whole again, he would have to leave. And he pleaded with Eru Ilúvatar in his mind, begged that highest of the higher powers to grant him peace, to fill the holes in his heart, to bind up his broken soul.