Take baby steps. While you are working on your current project, get out the pattern you plan to work on next. Then, in a day or so, start gathering the fabrics and playing with them just a little. Next, organize the prep materials-cutting board, rotary cutter, ruler, etc. Then, see if you can do the cutting on just one piece of fabric. This way, while you are working on something you may have grown tired of, you can dream and work toward the next project without actually having started sewing it.
I realize this may not be realistic. I have the opposite problem, I can start and have 20 projects going at the same time and it doesn't bother me.