Who sings the original version of this song done by jessica simpson - 'I want to love you forever' ?

I don't think anyone did. From what I've found, it's an original. But also, it wasn't written by Jessica. It was written and produced by Louis Biancaniello and Sam Watters. You can find that info here - I Wanna Love You Forever - Wikipedia, the free encyclopedia

Yeah, so as far as I can tell, the words and music were done by Louis & Sam but performed by Jessica.
Also, the commercial single from I Want to Love You Forever was released on the 28th of September, 1999... therefore, I'm pretty sure it was the original :)

You might be thinking of the song "I Think That I'm In Love With You" by Jessica Simpson. The original music came from John Cougar's "Jack and Diane".