And blaming him for not trying hard enough without seeing the effort he actually put into it is the same as blaming him for failing.
He blames himself! And gonna call stalemate on the "telling her about applying to her high school thing".
And I think lacking resolve (thanks for that word) is probably the better word/encapsulation instead of cowardice. But all this to say, his insecurity (which he's admitted) and lack of resolve (that probably stems from the insecurity) is why some folks don't like him
as a match for Hikari. Which like that insecurity and lack of resolve is why Aya hit on him and he accepted her so easily - your fave ship doesn't happen if he was secure enough in himself to have the internal fortitude to pursue Hikari. Eta: the example of this is him firmly turning down his Kouhai b/c he's set on whichever girl he loves, which yes would point to some character growth.
There was definitely miscommunication later from Yami's side (in how she kept her deep affection for him to herself, playing the above mentioned role instead),
Also cause I missed this the first time, flagging this not to argue w/ you but on the off chance impressionable teenagers find their way here:
This isn't miscommunication, it's a trauma response. She doesn't feel she can tell Yuu how much she cares for him, probably 'cause (as she's mentioned) all she sees at home is her mom getting hurt by the relationships she falls so deeply into.
A major reason this relationship is unhealthy is because sharing feelings is a
foundation of healthy relationships, and this is true whether the relationship is romantic, platonic, familial, or even professional. Sometimes it needs to be initiated by the other person - a parent saying "hey, you've been off, what's going on" - but Yuu doesn't have the experience or maturity or resolve to handle that kinda situation effectively, and also I don't think he knows Aya well enough to flag that something is going on.