So you're writing along steadily, and then -- you come to the end. Dangerous time. Doesn't even matter if you know it will only be a draft and needs more work later.
Switching between projects is a help there, but only so far. After all, you were working on the project that was alive to get it done. . . . If another project is jumping up and down in eagerness, it can help, but it can be dangerous to start a new one just because an old one's gone to the backburner, since it's going to have to come off the backburner at some point.
It helps to keep on writing on something else the same day rather than jumping about in glee.