The events of FFX-2 don't change what happened at the end of FFX. The end of FFX still happened, and is still as sad as it was when I finished FFX-2. It's just that when FFX-2 was introduced, the end of FFX just became the midpoint of the FFX saga.

I compare this to the Lord of the Rings books. The first book ended an a rather sad and disheartened note. And then you read the other two books and everything is happy at the end (well kind of). But that doesn't change the end of Fellowship of the Ring, it merely extends it. That feeling that you felt at the end of the first book isn't thwarted by the advent of the other two books.

The end of FFX-2 in no way ruined or changed the end of FFX. Tidus's journey and Yuna's journey were seperate, greatly intertwined, but seperate.