I used to be like that and I wasn't getting quilts DONE. I too would go onto something else when I got to a point where I could see what it was actually going to look like.
For the last two years I have worked on finishing things before I start something else. I did pretty good and got 12 tops done last year and several of them quilted so far.
I just keep telling myself that I can't start anything else until I finish what I'm working on. It can cause periods of not working on anything because I refuse to start something else but don't feel like working on the current one. I'm doing a lot better and I only have a couple of things going right now.