Have you tried in open tunings or standard? I started in open and then moved back to standard. When I first tried slide years ago in standard it didn't make sense at all and I think I got as frustrated as you. It really did help to learn all the classic licks in open tunings (where they were, for the most part, originally played). I started in E. (E,B,E,Gsharp,B,E)
I play with the slide on my little finger which frees up my other fingers for fretting chords OR (more importantly) muting behind the slide. There are players like Derek Trucks who supposedly don't mute at all. I'm not sure how they do what they do then. I need to in order to control the notes that sound. I also do fret notes behind the slide, especially in standard tuning, because there are a few things I can't do exactly that I need that little advantage to be able to make happen. For me that was the first big challenge...being able to play single notes and have them play clean and not messy, and then to find all the notes that can ring out on adjacent strings along the way.
In standard tuning, the first thing I also started with is figuring out where all the A based triads (the standard A chord in the second fret E,A,Csharp) were on the guitar in all keys. Recognizing this helps you have an anchor tonally in all keys. This helps you find the 5th, root and major third (low to high). From there I learned some licks where, starting on the root note in this position for whatever key I was playing in (always on the G string), I dropped down to the flated 7th (always a whole tone lower) and then back up, sometimes catching the minor 3rd and then sliding back into the major 3rd. In this position, there is also another root three frets up on the E string too. The fifth is also on the adjacent note on the B string. You can start here practicing all those old Elmore James licks and then dropping down to that A position triad and sliding into the end of those licks on the major third by sliding into it from the minor third a half step lower. The lick I am talking about is the "Dust My Broom" lick. You can do it in standard tuning although you have to move quickly to catch the last part. This gives you something to start with anyway. I hope this helps. E-mail me if you have questions.
Mark