My problem was collecting patterns, even though I had several projects going too. I have so many patterns that five people won't live long enough to make all of them. I just had to discipline myself--I can't buy or print any more patterns unless I'm going to make it right then--right now. Same can go for you. Pick a number of projects you have to complete before you can start a new one. You don't have to finish all of them--just some of them--and then discpline yourself! (Not trying to sound like your mother--but this is how I have to talk to MYSELF if I'm going to stick to my goals.)
Another way is to give yourself a challenge--weekly or monthly, or even daily, if you dare--decide how much you want to get done and then do it before you can move on to something new. For example, "I will do the quilting on this quilt by the end of the month."