After years of playing pretty basic guitar - chords with fingerpicking, I decided I wanted to be able to pick out the melody on the strings so that I could incorporate an instrumental break into my songs. I found that I would reach a point where I was terribly frustrated and just couldn't make it past a certain point - and I would leave it for a week or so and maybe work on another instrument (I play around with the Appalachian dulcimer and the banjo). To my amazement, I found that after a break I would often be able to play what I hadn't been able to before. Like it somehow sinks in when you're not thinking about it. Try just taking a break when you get to that point, then coming back when you're refreshed. It's been working for me and I've been improving slowly but steadily.