So from the top - let’s readdress her as “prospective partner” as opposed to “best friend”. It is incredibly obvious she is a love interest and you are only waiting for her to be available before you work your way in formally - you’ve been laying the groundwork for a great many months and are only waiting for the green light.
So this prospective partner speaks to you incessantly for over a year while working out issues with her boyfriend and you see no issue here? A woman is dating a man but sharing the bulk of her time exchanging intimate life details with another man until 2 or 3 am some nights, but you look at this individual and say, “yeah she’s got a lot to offer.”
She sees that you went on a date with another person and she cuts you loose and somehow you’re confused. I’ll clue you in - you’ve been formally dating this girl for a long time; just in secret. You exchanged ideas, opinions, preferences, desires, hardships, plans etc... you’ve built the emotional foundation necessary to accommodate an in person relationship and she’s been using this time to distance herself from her partner and work her way into your heart before she ever even leaves his bed. She’s covering her bases, this one.
So then you notice she’s not really as invested as she was. She and the boyfriend split ( her hearts already a mess might as well move him the fuck out in all the chaos ) she’s leaving you hanging and giving you one word responses and you are wondering why. Can we redirect the questions being asked here and draw attention to the questions that matter? Does it not at all concern you that she was actively and knowingly betraying the individual that holds the relationship role you sought to gain? Why would you want to be the boyfriend that she’s cheating on? You think she won’t? Then you are a fool.
It is interesting to me, however, that she is punishing you for the very transgression she, herself, is committing. She doesn’t tolerate you seeing anyone during your courtship however she has a full blown formal relationship in place and is doing to him even more damage than what she accuses you of doing to her. This girl is blind to her hypocrisy and you are blind to her character faults. Fight hard to win her heart because with as misguided as you both are, you absolutely deserve to be with one another.