It may seem insignificant, but start with a lot of 15 - 20 foot putts. When you make them all, move back another 3 feet. Many young players start off trying to make 30 - 40 foot putts without mastering the short ones. On the short putts, you'll fine tune your putting motion - a little variance left, right, up or down will matter because the flight of the disc won't compensate for the wrong release. Also, when you know you can make all your short putts, you won't worry so much about missing from 30 - 40 feet knowing you can make your come-back putt. You'll start making more of the longer ones because you'll stop thinking about "not missing" or where it's going to land when it misses. You'll be able to focus on making this putt.
Eventually the putting motion won't be the problem, but rather the motivation will. I putt a lot better when playing a game like D.I.S.C with someone else. Then you fine tune the mental aspect of the putt like I mentioned up above.