To be fair, he regretted it very much, but the damage already been done. CSS is not the type of person easy to forgive, so LBY had a lot of kissing up to do.
Another reason that the author let the two of them take time to reconcile (es. on CSS's part) because CSS needed to find herself, her values, her most important things in life and learn how to love. She was already in love with LBY, but she herself did not understand love fully, only toward the end of the novel, she would fully understand her love for LBY when she realized she could sacrifice her life for him.
In the beginning, she always wanted to find her independence, a marriage where she could be herself, because she was always restricted and constrained to do what she wanted. Love did not seem that important to her, she just wanted mutual understanding and respect between her and her future husband.
At the end though, she learned that being in love with LBY did not mean to lose herself, but also ready to sacrifice for the one you love, even at the expense of yourself. That is when she realized how important LBY was to her.
I really like how CSS took time to discover herself, instead of reconciling with LBY right away. It makes the ending sweeter and more satisfying. That makes the story so unique and unlike others, since Love like the galaxy is not only the journey of CSS to find love and happiness, but it is also a journey of her finding herself.
It's my opinion after reading the novel, I really hope they will bring these nuances to the drama as well. They really bring the novel alive so far.