The reason Claire doesn't believe her is one because of the comp het Claire goes through which prevents her from acknowledging her bi-ness. There's also the class divide where Claire is always thinking in the back of her mind that the only reason Rae is there (and in her mind teasing her) is because she wants something from her as a noble, not a person. Two because while Rae does declare her love, she's always pushing for "Claire's happiness" even if it's with Sein/Thane. This comes off as very hypocritical and always gives Rae a way out. She doesn't believe Claire can actually love her and is always also keeping Claire at an arm's length away so she has an out to being hurt. Three, it takes a longggggggg ass time for Claire to come to terms with loving Rae. Even though she's clearly in love by the halfway point she can never fully say it even though she gets jealous easily when other women start making moves on Rae. It's only at the end where Claire is able to put Rae above her duty to society.