An excerpt from Berklee professor of guitar Robin Stone, from a forthcoming Rock Guitar Improvisation course that I've been editing for Berkleemusic.com:
"Musicians are creatures of habit when it comes to learning new concepts and as the saying goes, 'old habits die hard. Musicians who have been playing for a few years tend to play within their established comfort zones. They know some tunes, have some licks down, and for the most part stick to what they already know. The only way to evolve and progress as a musician is to explore those areas of your playing that you don't have together. I always suggest to my students here at Berklee that every time they practice, they should work on the material they want to work on for some of the practice session but also pick one topic or concept that they are not well versed in and work on that as well. A typical example would be a rock-based student learning a jazz standard. Practicing things you already know is a good idea—we all understand that repetition is an important factor in one's playing technique and vocabulary. However, learning new concepts, though frustrating at times, will advance your playing beyond your current comfort zone and keep your improvisational ideas fresh."
Remember to add "new" skills to your routine on a regular basis... but don't just dabble with them and then start something else that's also new. Keep them in your regular routine until you've mastered them, then add something else that is new.
Just like "there's no such thing as very pregnant" -- there's no such thing as "half learning." You've either learned it or you haven't learned it yet. You can be working on learning, but no such thing as "half learning."