If you are passionate about learning to do something, you don't need discipline, unless that discipline is to stop and do something else for awhile. If you aren't passionate about something, no amount of discipline will make you great at it.
The best way to learn something is to immerse yourself in it. Read books and magazines and blogs about it. Participate in forums dedicated to it. Incorporate it into your social life. Watch movies and documentaries about it. Do it as often as possible in as many ways as possible.
If something comes easy for you, you aren't working on the right parts of it and you certainly aren't improving. You want to find the boundary between what you can do easily and what you can't do at all: you can do it, but it takes effort. Focus on that until it becomes easy, and then push into the boundary again.
Imagine a ladder where the rungs are labeled from lowest to highest: horrible, terrible, awful, bad, meh, average, above average, good, great, terriffic, amazing. You don't get to skip any of those rungs on your climb up, and the higher you are, the longer it takes to climb to the next rung.
Focus on the process instead of the end result. Just write; don't worry about publishing your novel. Just paint; don't worry about your gallery show. If you're passionate about something, you love doing it; having done it is irrelevant.