What do you do when you grow out of a person? I no longer have that love for someone I loved so much.
I would say that is a natural part of life. We change and other change, and these changes sometimes lead to having different opinions about the person, or no longer enjoying their company, or no longer liking them.
I don’t know if there is anything you should do about it. Just continue being polite toward them, and if they expect love and kindness from you, try to show it to them even if you do not feel it. As discussed in this previous answer, being compassionate when you do not feel like it is actually greater in virtue than being compassionate when you feel like it.