The glass on my iPhone 5s is separating from the digitizer.

andrienalinerush, could be caused by a low quality replacement screen. Of course it is also possible that it is a factory screen that missed the QA process. If the phone is still covered under warranty, I'd get it repaired that way. Otherwise consider getting Apple to do the repair for $129 If you can not do either because of geography or any other reason, you can always do it yourself. Here is the guide for it and the part is available right here

The top of the display has to be kind of "slid in" first. You'll notice little tabs on the top of the display that are meant to sit under the top of the frame. Then lower the rest of the screen and screw in Pentalope screws.