Girl, footsie... Meaning?

So, there's this girl, that I've grown to like in the past couple of months.
Everytime that we're in a social situation, either with friends of ours or just the two of us,
there will always have signs that she's into me (I'm into her aswell).
The first indication of interest were things like, prolonged eye contact, laughing etc...
there were a lot of touching from her part. She would always come near me whenever I would say something, of when I'm showing something on my phone, or she'll come near me just because we are together.
And speaking of touching, she started playing footsie with me, 2 or 3 weeks ago. Either we are sitting on a sofa, and our bodies, I mean our whole body would just make like one (elbows to the legs touching), or if we are sitting on a table, working or just talking with friends, she would play footsie with me. And I'm talking about the real deal. Meaning that her whole foot would press relatively hard against my leg.

As I'm writing this, I realise that this girl really is into me. And you, the person reading this, might agree with me. Maybe you're asking yourself: "Why do you keep asking yourself if this girl likes you or not? It's obvious she does".

Well, my excuse, if I can call it that, is that she has a boyfriend for over a year and a half and that they live together.

That is the only reason why I haven't tried anything with her yet. But why? I don't know.
Maybe it's a way for me to hide behind an excuse so I don't have to try anything, maybe it's the fear of rejection... I don't know...

So, I'm asking you, what should I do?
Girl, footsie... Meaning?
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