At an Impasse

What I would like more than anything right now is to write a decent short story, but unfortunately, I am stuck. The problem lies in my indecision. I'm attempting to write my first sci-fi short story and submit it to a magazine or perhaps publish it on Smashwords like my other short stories but this is something that doesn't come naturally to me.
First of all, I had a very diminutive idea of what is included in science fiction. I thought it was all about aliens and time travel. That perception was rectified when I did my research. And that was when I came across an idea that I thought I could write.
Then (this counts as secondly) I found that what twist I wanted to incorporate in my story was close to impossible because of the limitations in science.
Okay, so I could turn this into a sci-fi fantasy piece, but for some reason, the passion to work on an original story eludes me.
I need to be excited and surprised by my own story that so far I have attempted in my previous stories- particularly-Francesca and Eliza  and Horror Story.

I need to find something that draws me in to this concept and the plot I have concocted. It is a good idea but it is going to depend on how I write it up and for that I need all the passion I can muster to write.