With how it’s basically become obvious, to Momoka especially, that Rin doesn’t love her. It’s even more ironic when she says the words “I’ll give you all of me” when that doesn’t really included her heart or love and Momoka knows it.
Ah, but she does love Momoka. But just like Momoka, she's afraid that admitting this might drive her away again. The wrong idea, sure, and indeed not what Momoka would have wanted to hear, but the way Momoka herself has been acting, you really can't blame Rin too much for getting things wrong either. The two still have a lot to learn about each other and their feelings, but I see no reason why this should not work out in the end, provided they eventually manage to properly talk things through.