also I don't get what Yuu's thinking
Yuu doesn't want to admit that he
is feeling lonely without Tsubaki's constant company and that's the problem!
He thinks he's self-centered for wanting to spend time with Tsubaki, considering he's the one who said they can't hang out much anymore because of his studies... When Tsubaki's visits became shorter and less frequent, he started to miss him, and he probably feels bad because technically they're not in a relationship and it's not his place to want reassurance when he didn't give Tsubaki an answer yet. Even if he already knows how he'll respond...
Honestly Yuu should just give in and make it official with Tsubaki, then he can stop being so worried... He was anxious that Tsubaki will be interested in Shima-san, not to mention he feels like Tsubaki's slipping away because they've been meeting less often. Clearly he hasn't fully realized the extent of Tsubaki's feelings yet... He just needs that affirmation that their feelings are mutual and Tsubaki's not going away anywhere or anytime soon!