I think that Belly is still not seeing the whole picture. Conrad does love her,but she has to understand that his mother was extremely sick and has recently died, there for he has lost part of himself and needs time to recuperate because obviously not everything is gonna be the same anymore. The real person hurting from all of this is Conrad. Belly treated him badly at his mothers funeral by telling him to go to hell. That's just something you don't do. She should have been there for him, and be his rock. She was suffering from Susannah's death as well it was both of there jobs to be there for each other. Making the scene at the funeral was uncalled for. Conrad didn't go after Belly after she walked away because it must have been hard to lose his mother losing his love was probably something he just couldn't deal with.
By the way Conrad was acting I can see why Belly would think that Conrad has lost interest in her. Conrad is in his first year of Collage and she's still in high school, so I can also understand why be insecure. In that circumstance there's a possibility that he could be cheating, but it's not the case. His mother has passed away he can't think straight he's completely out of it. Right now he feels like his life has lost meaning. Being the older brother, and his dad crying every night he probably feels like he can't show his emotions. One, because he doesn't want to make it harder on his father, and two because he probably feels like he has to be strong for his younger brother. Him being in collage too must make it more stressful because of all his classes as well. With everything going on Belly can't think straight either. She feels like Conrad has lost interest in her because he seems distant,but that's normal because of what just happened to him. If she really did love she would be mature about it, after all he is older than her and might see things differently. She can't just think like a high school girl. She has to let him be he knows what he is doing. If he left it was because he needed to think things through.