So many people that I work with have trouble setting boundaries. It is understandable in many ways. So often they grew up in households where they were not taught, such as in the case of neglectful or chaotic homes. If the adults around you are not providing modeling of how to set boundaries and are not attentive enough to teach boundaries, then why would that child grow up knowing how to set boundaries.
In other homes, some of them were actively discouraged from setting boundaries. This happens in homes with narcissistic parents, where the kids are there to serve the needs and wants of the adults and are not rewarded for or taught to consider their own wants and needs. And of course, this too happens in homes with active physical and sexual abuse, where trying to set boundaries can actually be dangerous.
One of the things we do a lot of work on in therapy is learning to recognize where boundaries are and learning how to set them. And often the worry I hear is that by setting this boundary others may be upset. And this is absolutely true. Systems do not like change. But it is important to consider why that person is getting upset and often upon reflection it becomes clear that the other person has learned to take advantage of the fact that you have no boundaries and so they are naturally going ot be upset when those boundaries start to be enforced. But if the person truly cares about you they will learn to adjust. If not, that is good data on what type of person they are and how they see the relationship.