To properly articulate your unique value prop, you need to truly understand the value you are providing to users of your product. I know how difficult that can be when you're too close to your product. About a month and a half ago, I was in the same position.
Luckily, around that time, I heard the CEO of FreshBooks speak on the subject. Here's the gist of what he said:
Be able to tell users what life was like BEFORE your product and what it's like AFTER your product. Quantify it to make it real!
For example, if I've invented a teleporter...
BEFORE: On average 45 minute commute to work
AFTER: 0.015 second commute
Obviously, the big benefit of my invention is saving time. For yours, it could be cost, time, etc. Just make sure it's quantifiable!
And from there, writing your one sentence UVP will (hopefully) be much easier.