On Erynn's end, I'd say she is fully aware of how she feels. Sure there's a nervousness in her intimate moments, but why? MC clearly has his thoughts we can read, and he more or less fully admits to wanting more. Neither just seems to want to admit to the other what they actually feel.
The longer the story has gone on, I'm less inclined to believe it's the Ethan story that's keeping her internalizing. There's very little thoughts of hers that we (game player) are privy to, that makes me thinks that's it's related to Ethan in any way.
I'm thinking that it's fear. She expresses concern at least once, possibly twice?, that she's scared he'll take off again. She was already hurt/devastated when he cut her out of his life the first time and left town. She says as much when her and MC are in his room and she asks why he hasn't decorated his room, and that she's concerned/worried he'll leave again. As it's something she actually says directly to the MC, I'm thinking it'll probably be the, if not one of the, top reasons.