My thoughts are similar to Joe Guimarello’s posted here: take a healthy look at what is bumping you at the 80% speed, for example. Is it a particular technique that is giving your fingers difficulty? Is it more difficulty with the speed and precision of your left hand finding the correct notes in time? Is it your right hand getting caught up moving from string to string?
I’d say first: don’t get discouraged. I’ve been playing (though, not consistently practicing) for over 30 years, and I’ve been in that same situation as you a lot. Being able to identify what is giving you trouble in a specific song or solo is a great insight as to where to spend your next several practice sessions. For a long time, my issue was string skipping with my pick. So, I put a particular solo on hold for a few weeks and dug up practice parts that helped me work on string skipping… skipping just one string at first, then speeding that up, then slowing down again and skipping two strings, speeding that up, etc. After a few weeks of working on that, I was better equipped to go back and tackle the solo or song that was difficult.
Also, when starting at 50% speed: ask yourself if you’re starting to ingrain into your muscle memory “mistakes” (by mistakes, I mean things that will make playing at 80% or 100% speed difficult). Is your thumb in the best spot on the top or back of the neck to do what you need when you start speeding up, for example? Could the notes you’re fretting be played faster in a different position up or down the neck, depending on what’s coming up in the next few bars of music?
Ids Schiere’s idea is great, too – sometimes taking a new direction, even for a little while, can bring some fresh perspective.