Why is Writing a New Genre so HARD?!

So, I’m a primarily romance author…. or I’m only a romance author. I wrote a “survivor” story once, and it was weird to not have romance in it. Everything I write ends up having some kind of romantic spin. I love romance. I love LOVE! So, why in the world am I trying to write a new genre? Not just any new, but a complete 180 from my norm. I’m attempting a Thriller/Suspense story.
Romance is easy. Boy meets Girl; one falls for the other, or they both fall together. Lots of S3X and usually a happy ever after. Plus, who doesn’t want to escape reality for a while and enjoy some love and spicy scenes? I am even an avid reader of romance! I will read pretty much any sub-genre of romance.
And yet, here I am trying to write a Thriller… It’s totally different than romance. I am finding I need to actually plan and plot this story. I need a good opening scene to get readers interested in actually continuing this story. Romance is driven by the relationship and the what ifs of the story. Thriller is driven by the who done it. I’ve got the general story idea figured out. I know who did it and why. I know what they did and to whom. I know the characters and their roll… BUT I DON’T KNOW HOW TO START IT!
There are so many options with a Thriller. Should it start in the present with a fade to past? Should it start at the beginning? Should it start with the crime? Should it be 1st person, or 3rd person? Should the MC be the victim, the detective, the bad guy, some other entity? SO MANY QUESTIONS!!
I don’t have a ton of author friends that write this genre. Most of my friends write romance. Even Thriller Romance. I don’t want romance in this story. I want suspense and wonder and on the edge of your seat, page turning THRILL!
HELP!!!