OOH! Yay! I love these types of books too!
Ok. So here is my list:
Blue Bloods by Melissa de la Cruz.<--this isn't exactly just a love story
but it is really good. It has all the romancy stuff too. though! And only does it end unpredictably(is that a word?) but it has a cliffhanger.Plus it has to do with the irresistible:vampires!
And it's sequels:
Masquerade
Revelations(coming out oct. 10)
Uglies by Scott Westerfield<--it's not just a love story either but it is a sci fi that incorporates all the life and love of the characters.
And it's sequels:
Pretties
Specials
Extras
Blue is for Nightmares<--Again, doesn't focus on romance but as it is about a teenage girl who lives in a coed boarding school, you can expect some. Also has to do with witches and such if you enjoy that.
And it's sequels:
White is for Magic
Silver is for Secrets
Red is for Remembrance(I'm not positive about the order)
Daughters of the Moon Series<--This is really something that doesn't cover any of your criteria for books that you would like but I have similar tastes to yours and I really like this series. There is like 12 books in the series and I don't know the names of all of them