There are three aspects to your story’s main plot, and the good news is, you’ve already identified them.
The first aspect of your story’s main plot is the external conflict, the problem introduced by the inciting incident, which the protagonist then sets about trying to resolve. The action they take in trying to do so and in overcoming the obstacles that get in their way drives the plot forward.
For example, in To All the Boys I’ve Loved Before, the external conflict is that Lara Jean discovers her sister’s ex-boyfriend, Josh, has received the love letter she wrote him years ago. She attempts to avoid him, as she cannot betray her sister by talking to Josh about her feelings for him.
The second aspect is the main relationship. All good stories are built upon the emotional stakes of a relationship, and in a romance, the main relationship is the one between the protagonist and the love interest. This relationship should be directly connected to the main external conflict in one of the following ways:
Whatever connection the relationship has to the external conflict, the protagonist will ultimately have a goal for this relationship, even if they don’t articulate it or admit it to themselves. Usually, this goal is simply for the relationship to work out.
For example, in To All the Boys I’ve Loved Before, the external conflict motivates Lara Jean to strike up a fake relationship with Peter in order to misdirect Josh. The more time they spend together, the more she begins to feel for him.
The third and final aspect to the main plot is the internal conflict. To combat the external conflict and because of the main relationship, the protagonist must grapple with internal conflict. They must learn important lessons during the course of the story in order to overcome their misbelief and find happiness.
In To All the Boys I’ve Loved Before, Lara Jean must learn to open herself up to the possibility of love in order to overcome her misbelief that the more people you let into your life, the more that can walk right out of it. In doing so, she finds happiness with Peter.