Why doesn't my girlfriend think I'm attracted to her?

So I've been with my girlfriend for a year and half, and I personally think we are very fortunate and have a great relationship. Our one problem however is that she just doesn't believe that I'm attracted to her. If I don't initiate sex with her she gets mad and accuses me of not liking her. And the other night we were trying to get intimate and I just couldn't get aroused. I told her it was because I was exhausted and had been drinking. She does not understand this and seems to be willing to end our relationship because of these issues. Please help me explain to my girlfriend. I'm very attracted to her, I don't know what else to do to convince her.
Why doesn't my girlfriend think I'm attracted to her?
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