ask yourself this .. is it really love? or is she really worth it? ...
you could tell her how you feel but that might seem to confuse her;
You say she knows you but has she gotten to know like the real you inside? what if she has feelings inside for you but is scared to to tell you because she might end up hurting you and her boyfriend. If she does has feelings for you i suppose she could be playing with your head + feelings aswell... you have to talk to there but dont be so clingy niether , like ask her normally what her thoughts are about you.
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