Although I’m not from the US, I would like to point out 2 possible reasons why they don’t treat you the way they did.
1. You said: When I was 15, I came to NYC for the first time, right? In some of your questions you seem to indicate that you aren’t a born American, so maybe the people in NYC saw you as a newcomer, and of course anything new will attract people’s attention. Your look might be different from anyone they had seen before, and probably attractive too. That may be why they took a special interest in you. And now you have grown up and probably have lived there for a long time (or at least you have lived in the US for a long time), so people get used to you. So although you may not changed, people have lost some of their interest in you.
2. Like most jelly here has stated, you have grown up, and that makes it more difficult for people to approach you and say something like that to you the way they did in the past. You are 20 years old, right? So you must be able to distinguish the difference in how typical people praise a kid and an adult. What kind of adult would approach another adult and say: “Oh! You’re so beautiful! Beautiful!”? What do you think the adult who receive the praise will think? Does the compliment make him/her happy, or does he/she think the compliment is embarrassing and childish? This whole scenario looks like when an adult claps his/her hand and cries out loud: “Wow! You’re so good! You will win an Olympic medal!” at a child who is trying to learn how to walk.
I really can’t say for sure whether you are uglier or not, because I haven’t even known who you are. But don’t just think of yourself as ugly. Have confidence in yourself. The confidence only comes from yourself, not from others’ compliment. If you think you’re beautiful, you are beautiful, no matter what people say. Don’t just judge yourself based on others’ opinion like that!
A saying for you: “There’s no such thing as an ugly woman, there’s only a woman who doesn’t know what makes her beautiful”