Jill has not been established as a decisive, strong character. She's trying hard, but she has inner conflict and it's been displayed numerous times when we can see her inner thoughts:
When she see's the mc's cock in the mirror:
ji "(Wow...)"
ji "(No! Not "wow"! Why did I do that?)"
ji "Fuckface...?"
mc "Yeah?"
ji "I'm sorry...but the mirror...I peeked."
mc "You did? Haha, why are you telling me that?"
ji "It was an accident, but it still feels so wrong..."
ji "It's so not me."
When she starts making out with the MC on Bella's couch and then stops, saying it's going too fast:
ji "(I'm so stupid...right?)"
ji "(No... That was smart.)"
ji "(It's way too fast for me...and this isn't the place for it either...)"
ji "(Bella's on the other side of that wall... That's so disrespectful...)"
ji "(But... He just makes me feel...so...)"
ji "(Gah! I can't think straight!)"
ji "(My head is rushing.)"
ji "(Breathe!)"
When she's struggling to play the piano:
ji "(I'm not good enough to do this...)"
ji "(I can't focus today.)"
ji "(You know how to play this. You've done it hundreds of times by now.)"
When she's not in a strong relationship with the mc and decides not to call him when she's struggling with the piano:
ji "(No... Don't call him.)"
ji "(I'm not even gonna tell him....right?)"
ji "(...)"
ji "(Focus!)"
ji "(Try it again. You know this!)"
ji "(I suck!)"
When she is in a relationship with the MC and he shows up when she's struggling with the piano:
ji "(Tybalt isn't right about him. I refuse to believe that.)"
ji "..."
ji "(Just tell him...)"
When the mc turns her down when she comes over for the impromptu birthday picnic:
ji "(I can't blurt it out now... Crap...)"
ji "(Maybe it's a sign?)"
ji "(Maybe it's best to not even tell him...)"
Even her actions show she is conflicted, like calling a meeting to discuss Chad bullying the MC (in spite of the mc's protests against her doing anything about it) and then running to Bella to cry when she realities she might have done the wrong thing.
And even when she's talking direct to the MC, she sometimes stumbles with her words and corrects herself.
All of this is deliberate characterization implemented into the story by DPC. If you don't pick up on these (not so?) subtle aspects of a character when other people do, you can't blame the author.