I always have multiple projects going at any given time. A few get put aside for literally years, but I tend to get something done just about every week or two. I think I do my best work when I'm feeling fired up about it. I also like to have a few things going that are "promised," maybe not in the sense that the person who will get them would know, but I've promised myself to get them done in time for a birthday or promised myself I'd have a donation quilt done in time for the next guild meeting, or that sort of thing. I have found that if I do get something done fairly regularly, it helps to keep me from getting discouraged about the things that are not necessarily moving along on a noticeable schedule. It's good to have a few projects that can be finished quickly, just to assure myself that I'm not hopelessly floundering. Like you, I have done all sorts of crafts over the years, but I tend to focus on one at a time.
One technique that I learned from one of my kids' swimming coach years ago is to mentally visualize the project progressing to completion. It's a matter of thinking it through, down to tiny details of each step. I find that a lot of times when something stalls it's because there is a decision to be made, such as what kind of border is needed on a quilt, and until I see something in my head and see how to do it, the next step simply will not happen.
As others have said, it does feel good to finish something.