I don't know if I can help with the ending but I can help with what we have here. Let me format it a bit for you.
First off arduity is not a word, i think what you're looking for is arduousness although it would sound awful in this context. I think it sounds awful in any context to be honestt. Why not use a synonym?
The harshness of my memories of you,
is much greater than what it seems.
The agony I feel has turned me selfless,
Words will never be enough,
for you have turned heart to stone.
How does that sound?
Kinda bland I know but it does make much more sense.
well i would say it sounds good , check the defs once more with sources and make sure it says what you wanted to or are willing to state, per a Ross-ian accident, then make the stone happy and make it either sink into a dark line, not morbid , dark, or float like a balloon with muse...