Well, I've heard that Stephenie Meyer's Twilight books are supposed to be great although I've never actually read them. Have you ever tried Meg Cabot? She is the author of the Princess Diaries (my guilty pleasure) and those are actually pretty cute and funny.
You say you like mystery and thriller, but how do you feel about supernatural mixed in with that? I love the
Mediator series. From amazon: "The inaugural installment of The Mediator series introduces high school sophomore Suze, who, in her words, has "this unfortunate ability to communicate with the dead." As a "mediator," the girl helps ghosts put unresolved issues to rest so they can move on to the next world." It's also got romance as she ends up with a crush on the ghost who lives in her bedroom.
I'm trying to think of things that would fit just your preferences listed, but if you
do like books with a supernatural or fantasy element then the following might work.
A Great and Terrible Beauty by Libba Bray
Midnighters Trilogy
-Science fiction/supernatural; Some romance and mystery; Definitely thriller- I was on the edge of my seat so many times during these books.
Also, one I just thought of is
Waves by Sharon Dogar. It's not one of my favorites, but it's got a lot of mystery and romance as well as other themes.
"Grade 8 Up–I'm in a cupboard. A dark cupboard, and it's too small for me. The walls press against my flesh….Through a chink of light where the door is barely open, I think I can hear voices. So many voices. Help me! These are the unspoken words of Hal's sister Charley, lying in a coma ever since the previous summer's late-night surfing accident on a Cornwall beach. Now it is July once again and Hal's family is off to Brackinton Haven for their annual holiday, leaving Charley behind for the first time. Torn between his anger at his sister for devastating the family and his desire to discover exactly what happened, Hal hears her voice more and more often. As he gets to know the surfer crowd that Charley hung out with and begins a romance with the younger sister of Charley's boyfriend, Hal slowly begins to unravel the mystery. Told in a series of episodes with headings such as Charley. Then, Charley. Hospital. Now, and Hal. Graveyard. Now, the narrative skillfully shifts in time and point of view. Readers of Alice Sebold's The Lovely Bones (Little, Brown, 2002) and Peter Dickinson's Eva (Delacorte, 1989) will be intrigued by Dogar's exploration of such questions as: Where exactly is a person when she no longer inhabits her earthly body? Can she communicate with those she has left behind? Both suspenseful and thoughtful, action packed and atmospheric, this novel is compelling and memorable"