I think some people need a wake up call. Let's look at the girl you want:
1.Genuinely sweet/nice
2.Possible virgin
3.Doesn't (currently) do any drugs/heavy drinking despite being:
4.Between the ages of 18-21
5.Attractive enough that you feel attracted to her.
So.... basically you want every guy's dream girl. Let's look at each of these things in turn.
1. I don't know any guys that like b!tches, or fake chicks. Any chick who is genuinely sweet and nice, even if she's shy, will be drastically increasing her chances of guys trying to game them.
2. I don't know any guys who like their chicks to be slvts. Any chick that appears to be innocent, or at least doesn't appear like a slvt, will have many more guys hitting on her than slvttier chicks, all things being equal. Funnily enough, this also seriously decreases the chances that she will still be innocent(ish).
3 + 4. These two go hand in hand. I don't know any guys who like drugfvcked alcoholics, and every guy, even those in their 40's, all want to get with chicks around 20yrs old. Between the ages of 16 and 24 is when chicks are in "party" mode. Reality still hasn't hit them yet, and they still think they'll be young and hot forever. If any girl is ever going to be doing drugs or drinking excessively, it's going to be in this age range. And many, many girls do.
5. I don't know any guys who aren't attracted to attractive chicks (crazy, I know). Any attractive chick, no matter how shy (unless she's agoraphobic), will be hit on by guys whenever she goes out. If she's hot enough for you to be attracted to her, she's hot enough for other guys to be attracted to her.
All these things combined are the reason that attractive chicks who meet the requirements you laid out are extraordinarily rare. Not only is it unlikely that such a girl will exist, but it's also unlikely she'll be single if she does. Any girl like this would have guys on her like stink on sh!t. Every man here on this site would game her in a second. This fact alone would result in her very quickly becoming less "sweet and nice", because she'd have to develop the same b!tch shields that every other attractive girl develops.
If you meet a sweet girl, who you think is genuine, I promise you she's sucked miles of d!ck. I've met many, many girls who seem deceptively innocent, and it appears that the more innocent they appear, the more of a wh0re they really are. And I'm talking 17yr girls who've already fvcked upwards of 30 guys, but you'd never know unless you got to know them as well as I did. Their "sweetness" is merely a clever ploy, playing on the very fact that guys are attracted to this trait.
You only have one chance as I see it: Raid an Amish community.
Either that or lower your standards. You can find mostly nice girls who are attractive and who aren't complete alcoholics. They won't be anything close to virgins though.