i dont understand

I feel this pain,
deep within my chest,
she was my love,
and it was the best,
i tried so hard,
yet i failed again,
I dont understand,
what was my sin?
what did I do,
to deserve what I got,
I tried to treat her the best,
though i know i did not,
but what was my transgression?
God, what did I do wrong?
why did you take my happiness,
and take from my heart, it's song?

And please oh Lord,
how i cry for it back,
and now I feel,
that there is something i lack,
It's that safety and joy,
that I felt in my heart,
has been violently torn out,
but has left cupid's dart,
there torture and plague,
and remind me of my misery,
and I know she feels not the same,
revelling in her time without me.

But I dont understand what I did,
to deserve this pain I feel,
every night before I'd go to sleep,
I'd take down my rosary and kneel,
I prayed for love eternal,
and for her smile to always be on,
and eternal is my love for her,
but her love for me is gone,
and I wish she would comeback,
because I know I could make it better,
but I already packaged my heart and soul,
and sent it to her in a letter,
But it didnt sway her decision,
and when she told me what was the matter,
that she no longer loved me,
I felt her step on my heart,
and I felt my heart shatter.