You're the writer, so it's your call. That's one of the strengths of novels vs. movies, which are "focus grouped" to death. You have to write an ending that is honest. You don't owe the reader a happy-happy, joy-joy ending if it has to be forced.
Having said that:
Risen was traditionally published, then went out of print for several years. During that time I worked with a film producer on a screenplay and some nice changes came out of that collaboration. When I got the rights back and decided to release the book for Kindle, I changed a couple of scenes and revised the ending because I liked the screenplay ending better.
Of course, you don't just "change an ending." You have to change a number of things that lead up to that ending. I had to go back through the whole last third of the book to make the new ending credible.
So...tread carefully!
As for the title: I don't know what your title is, but I'd love to sell 1000 copies a month. Once you're through with the old title, may I have it?
