How to practice alone for a presentation?
Perhaps you are practicing the wrong thing.  If in fact you have no trouble speaking smoothly when you are front of a live audience, then you don't need to practice that.  You may need to practice how you 'enter' a slide (what concept or thought you will discuss first), or how to transition to the next slide (what is the final concept or thought, and how does it connect with the next thing you want to talk about).  You may need to practice what words you say in the introduction... you don't want to make that up on the fly.

In other words, do what professional musicians do.  They rarely play an entire piece through during practice, but instead repeat the challenging sections.  

With the goal of working out the words you will use in the tricky parts, practice will feel less artificial and more productive.  Don't sweat the pieces you already know.  At the end, do a dry-run with an audience to test the timing and the flow.