This is one of those posts where I'm going to tell you something that seems obvious but can be overlooked too easily.
So your protagonist is about to embark on a character arc. They will be changed, and much will happen to them over the course of your book. They might even fall in love. Great!
However. The protagonist shouldn't just step into the story and start existing. They're not there purely for the plot and romance. They should already have goals, desires, motivations, history. You know that, but do you know that?
Your protagonist should have a life AS WELL AS the romantic plot, and even as well as the main plot. A personal agenda.
The idea of doing this isn't just to give the character realism. I find myself more empathetic towards a main character who's out doing something besides what the main plot (or romantic plot) requires.
I'll give you some examples:
1. In Marissa Meyer's CINDER, there were plenty of subplots, the romantic plot was with the prince, but Cinder's own agenda was to run away from her stepmother.
2. In HEART'S BLOOD by Juliet Marillier, the main plot was about the curse, the romantic plot was with Anluan, but Caitrin's personal agenda was to escape from (and later face) her own demons.
3. In LEGEND by Marie Lu, the main plot was about uncovering the truth about the murder, the romance was entangled with the main plot, but Day's own agenda was to get medicine for his family.
A lot of people complained Bella from TWILIGHT had no personality. At least part of this stems from her lack of personal agenda. Stuff happens to her, sure. But what does she do in the meantime? What does she want? What part of her can I root for?
I've only realised this consciously in the past few days. The main character in my current WiP has a lot happen to her, and there's a love story in there too, but until the plot starts she does very little on her own. She has problems to deal with that turn quite dangerous... but that still doesn't give her the characterisation I'm looking for.
In my next draft I might have her involved in a small political campaign, or trying to start up her own business - something that tells the reader who she is, but also introducing a danger of failing (and therefore giving the reader something to root for) without it directly being about the main (or romantic) plot. It will be her personal agenda, and it should make a world of difference to the story.
Think about your current WiP's main character. What is his or her personal agenda? If they haven't got one, try coming up with something they want for themselves OUTSIDE the main and romantic plots.