You don't need an antagonist. You need some way to force the MC to confront herself. The usual way to do that is to provide an antagonist who presents exactly the right problem to force that self-confrontation. But that's not the only way.
A story is often three stories. The inner story is about how the main character is her own antagonist. The outer story is an important, meaningful problem on its own... but even more importantly it brings the MC to the point of confronting herself. The third story (typically a relationship story) provides yet another opportunity for the MC to see herself, and at the same time offers exemplars (good and bad) of various ways other people deal with similar inner conflicts. This relationship story is often where someone directly, pointedly, and correctly points out the MC's flaws.
It sounds as if your story doesn't provide that outer story, the one the brings the MC ever closer to seeing and confronting herself. That might work... as long as something brings her to that point.
One of my favorite stories is Kazuo Ishiguro's Remains of the Day. In that, I'd say that Stevens (the butler MC) is the main antagonist. There is another antagonist or two in the form of his boss's political friends, but the problems they create for him are few, and they are problems only because of his own choice to value his idealized (and obsolete?) sense of duty over fully expressing himself.
His main problem is how to address his growing mutual attraction with Miss Kenton. Again, the problem is entirely of his own making. She certainly isn't an antagonist in this.
All of these factors slowly but relentlessly drive Stevens into a corner (literally into a corner in a heartbreaking scene in the movie), where he must choose between the risk of human connection and the safety of solitude and duty.
So: You don't absolutely have to have an antagonist. But you will need some way to force the MC to confront herself.