I'm in a relationship but think I've fallen for female friend

I'm a guy in my 20s and have been dating my girlfriend for just over a year.

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It's good, but I've started to think that I'm in love with my female best friend, who I've known for years, as our mums are friends.

I think about her all the time when we're apart, even when I'm with my girlfriend. I also find myself comparing my girlfriend to her, as well as every other girl I meet, and no one comes close to her.

I don't know how she'd take it, though, if I admitted how I feel. We've been friends forever and nothing romantic or sexual has ever happened.

Plus, she's usually never without a boyfriend and could have her pick of anyone.