Try

Dear Niall,
I loved you more than anyone or anything ever in my life, despite what you did. and since I couldn't stop my true feelings, I have decided to do this to myself. Horrid, and maybe rational, but I loved you so much, I needed to stop. This was the only way out.

Please don't ever think I was a fool or mental, I was just a girl who couldn't handle her own feelings.
But... I loved you forever and always.
Samantha.