The cipher is a simple incremental cipher. Each character in the code is $L-n$ where $L$ is the numerical position of the letter in the alphabet and $n$ is the index of the character in the encrypted text.
I actually solved it by guessing the last line first, and then reverse-engineering that makes it quite obvious what the cipher is.
Working through the message, it decrypts as
I HOPE THIS PUZZLE IS NOT TOO HARD TO SOLVE
ONCE YOU GET THE FIRST FEW LETTERS, THE REST IS EASY
BUT IT IS NOT AS HARD AS IT SEEMS
It would be nice if you don't make it so so so long next time though ^_^