I just looked up at the reviews, the complaints points toward the same - the main character. From what I've gleaned upon, he's very flawed as a person, which is understandable given his background. However, people will eventually get fed up on a person who just doesn't want to help themselves, which I would assume is the problem.
Just gotta show your readers some promise for the main character to grow. Think about mushoku tensei (I'm hazard guessing it's where you took inspiration), and what made Rudeus relatable as a person.
It doesn't have to be as fast as rudeus' growth, but look as to how he grew. I mean, if the main character's problems keep piling up, the readers wouldn't be able to make up the end of it. Maybe try getting your main character face his problems and solve it one at a time, or maybe show a parallel to what he has become (i.e. by showcasing what kind of method he would use differently on the same problem).
Facing them one by one. Well, they're kinda related. The first tragedy gives him the trauma and the second shows him that his bravery on facing a stronger opponent he once had at the first tragedy had disappeared. So he needs to get stronger both phycially and mentally.