Well, here's the first thing...she's 20 years old. She's still a kid. At that age, every girl wants everyone to think they're beautiful. Unfortunately, not everyone eats from the same plate, so not every man will think one particular woman is beautiful.
Being female...well, what you said really wasn't right. You did basically tell her, "You looked better before than you do now." aka (in girl terms) "You look ugly."
Girls always read between the lines, even if there is no message there. As far as her dumping you over it...well, is she PMSing? If so, that could be the reason she jumped the gun on dumping you. Or she really isn't feeling you, so was looking for a reason to get out of the relationship by making you out to be the bad guy. Now she can go whine to all her friends "I dumped him because he said I'm ugly! What an asshole!" So, now all her friends will hate you too.
I wouldn't sweat it. Too many fish in the sea to dwell over one. Go for a female that is at least 22-23 and has gotten the 'bar scene' out of the way. Too many guys I know have been dumped when their g/f hit 21 & wanted to just 'have fun' and not be 'tied down'. 20 really is young.
P.S. I can't vote because I don't think you or her were either at fault. You both had your faults in this ending.
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