No, he already in love with Shizu long time ago. But when Yuki (Mafuyu ex bf) was alive, he like Yuki in a way of admiration. So he always look at Yuki coz he admire him so much. That's what make Shizu misunderstand that Hiragi love Yuki in lovers way. Basically he love Shizu more & different than he love Yuki. I would say his love for Shizu is deep but he never realized it before bcoz Shizu is always beside him. Now that they have a fight, he realized it.
PS / this is my way of understanding & sorry if it confusing bcoz I'm not English too.
Hiragi loves shizu since long before, but realize how much it is just recently. It is said on recent chapter that why he really fixated on yuuki was bcus he saw mafuyu&yuuki's relationship as 'perfect first love/love'. I think hiragi saw himself and shizu in mafuyu&yuuki relationship.
This is why he also feels conflicted with uenoyama and mafuyu relationship

I didn't understand... who was hiragi in love with? I get that now he loves shizusumi but I don't get if it's real deep love or the same love for the other person he mentioned (I didn't understand who.. sobsob)
Sorry for my lax in English but I'm Italian so plz if u know answer me I'm sad rn for the plot