Who sings this old song that is probably from motown and made in probably the 1960's that has the lyrics sung by the lead singer and the back up singer, "I love you, yes I do, yes I do" and then the lead singer immediately sings loudly, "It's so hard"?

Are you talking about the Rod Stewart song called "This old heart of mine (is weak for you)? It was first recorded by the Isley Brothers, i think.

This is what I immediately thought of. I figured maybe you were talking about when they come in with the words "This old heart of mine... " and thought it said "It's so hard... " sounds the same anyway.