Questions: Girl's Behavior

I am in a situation where this girl at work gives me all kinds of signals that she likes me. Whenever I go somewhere, she'd always catch me and smile to me a lot, being very sweet. I have noticed that she'd act such only to me. I also found her sweet, so I have asked her out for coffee but she was very hesitant and her mood and attitude changed towards me all of a sudden. She felt like embarrassed after that move. Then the day after she became kinda upset and turned off. I don't know. So then a week later, I have talked with her about why she was so turned off by me all of a sudden because it bothered me. I said that I felt jealous and insecure because she'd choose him over me etc. (Open communication). So she was sweet to me again. Wow... holy grail.

Also she seems to like another guy which made me feel jealous, insecure and very questioning. I know that such behaviour is a turn-off but it's mere human instinct and very normal. You just have to control the urge to seek validation by chasing. It's super duper difficult, I know. Everyone has had this feeling.

However, I have recently found out that she is actually taken after I have stumbled on her instagram page by coincidence. She had a name combination with a heart symbol.

So I assume that she already has someone which makes so much sense. Whenever I came closer to her (not physically of course), she gets turned off. And when I am absent and don't do the right move, she becomes disappointed and feels more attracted to me again.

Also, she seems to look at me very often in a curious and questioning way. I am so confused right now. And to the other guy, she is sweeter to him than to me. So right now, she is ignoring me now. It tears me up.

What I think: She is not happy in her current relationship, but is very scared of her boyfriend to say that open and directly to me of her situation. What do you think?