I have trouble with this sometimes myself. So, it occurred to me that writers (and many others) might be going through the same thing. You can't please everyone no matter how hard you try. This goes for the stories you write and the dinner you made last night. People will always find something to complain about. If you serve the dish to enough people, eventually someone is going to complain about the flavor.
Forget it. That's right, wash those old dishes (so to speak) and start cooking something new. Yes, I'm talking in metaphors and I've had too much coffee, but you know what I mean. Everyone cannot be pleased, it's not possible.
Someone will always think the story was too long, or too short, had too little plot, or too much plot, blah, blah, blah. The list goes on. It also depends on which day they read the story. Maybe they had a bad day or were too tired to follow the story, etc.
Now, if everyone has the same complaints about your book, then you may have a genuine problem. However, if random people crop up with random complaints (and they’re bound to), just forget it. I would say do your best to focus on your next project and move on. But, there is no such thing as try. Yes, Yoda would be proud. There is only do and do not.
DO NOT let negative comments get you down. DO move on to your next project with enthusiasm and a positive attitude. Remember, you've got something valuable to contribute that no one else has, YOUR words.
No one in the world can tell a story the same way you can, and people are waiting to hear what you've got to say.