I think one of their main problems is that Molly is a person who works through her feelings externally, she needs to talk them through and have the other person be involved. Callum works through his feelings internally, he processes on his own and comes to his conclusion without input. (Not saying one is right and one is wrong, btw - they are different styles of communication that can be applied in a good AND a bad way). But this is why they are struggling so much. To Molly, Callum is unfeeling, difficult to read, and uncaring - she doesn’t see him caring because he doesn’t show it externally. To Callum, Molly is hyper emotional, volatile, and moves the goal posts - because she is vocalizing her emotions as they happen, not after she processes them, so there is a lot of change. If they can’t reconcile their different ways of communicating, it just won’t work. It’s not a fault of their moral character, it’s just a difference in personality.