Am I just a "spare tire" friend to her?

i hate this feeling.. I like this girl and she knows it, but we still remained as friends now.. I often ask her out for dinner but lately when I tried to ask her out, she would agreed to me in the first place as she tells me she doesn't have any event on, but in end she would text me saying that she can't make it cos she is meeting her friends instead.. so what does this means? am I just a "spare tire" friend to her? does she really treat me as her friend? this is make me looks ridiculous..
Am I just a "spare tire" friend to her?
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