part 3 wew
Star's brain only becomes conscious of her feelings for Marco in BBtBC, specifically when Marco shows up in the suit and Jackie sees him. Prior to that, she's totally cool with Marco going to the dance with Jackie and is pushing him to do so.
When she sees him in the suit, her heart is basically fully in the "falling for Marco hard" stage and her brain is starting to catch up because Star is consciously finally aware "wow Marco is REALLY attractive and I like that."
And when Star sees Jackie's expression which is very similar to her own, she realizes everything consciously. That Star does have a crush on Marco and is kinda falling in love with him. This is the first time Star expresses doubt over her pushing Marco to Jackie and it's the first time she consciously acknowledges the depth of her feelings-as more than just a little crush because she found Marco kinda cute.
The rest of S2 is Star trying to deny her feelings and suppress them and do what is "best" for Marco by not telling him her feelings and continuing to push him closer to Jackie.
Star does this because she's telling herself that Marco is happier with Jackie and that she knows what's best for him (to be with Jackie) and that it is her duty as a best friend to ignore her own feelings and push Marco with Jackie. This is also totally wrong because it removes Marco's agency and is an insult to the friendship in a way, but Star's intentions are good. Unfortunately, emotional suppression is a bad idea and the whole "I must not tell my best friend I have feelings for them and push them with someone else" thing is counter-productive and a load of shit. Star does cause a lot of this situation to happen (i.e. a lot of it is her fault), but she genuinely had the best intentions for Marco (even though her logic was flawed).