Symptoms and diagnosis:
The editor complains that "the story ought to be gripping but it's so sentimental I want to puke."
The agent says, "I've been trying to sell it, every editor says it's way too sentimental, and I kind of wonder about my own taste because I like it so much."
Readers of all sorts, professional ones and supportive friends, say, "I was really into it till it turned all sentimental."
One way or another, every outsider reader slings that dreadful word sentimental at the work. And the writer says, "But how can it be sentimental when it's exactly what I feel? Am I supposed to write stories without feeling it at all? Or just be so cool that I bore myself? Why am I not reaching people with what I think is the most important thing in the story?"
When anyone tells you your work is sentimental, they are likely (but not certain) to be right. Many readers have excellent radar for sentimentality, in my experience. The trick is to understand what it is, nail its exact cause in this case, and see why it's presenting the way it is. Once you do that the fixes are obvious to the eye (but may be miserable to the glandular system).
This one took forever, even though it's the shortest one so far, because it's the least mechanical thing to fix. One person's emotional intensity is another's sentimentality, but some people have better judgment than others for it, and so there's kind of an uneasy three-corner-or-more deal between the writer, reader, and middlefolk (like editors, book doctors, critics, etc.) to get to that "enough but not too much" point. And explaining "enough but not too much" takes more thinking than just laying out a procedure that will move things along. So it was a battle the whole way to make the ideas reasonably clear, and I'm not sure I won.
If you write, and you've written a lot, you'll have been picked on at one time or another for sentimentality; see if there's anything over there that helps.