Hell, I'm guilty of it. Absolutely. I have a shit ton of things I'm doing at the moment that are intertwining with each other but I'm taking it bit by bit. It's never a good thing and you can definitely end up doing nothing at the end of the day but then, with a well cut out plan, it is quite feasible.
Sometimes, you don't plan to do that from the start, you start with one thing and along the line, you have to do another and you cannot afford to drop one task for the other. So you end up combining two things, which may become three or four tasks. Progress would definitely be slowed down compared to when you are solely focused on a single task and it can be quite demoralising. When you start feeling that lethargy, that lack of motivation to forge ahead, maybe it's time to take some actions.
For instance, you have to understand your capacity. The number of things you can handle as a person. This actually varies from person to person so you would have to figure out that on your own- who knows you better than yourself? You may redefine your tasks/goals. At least, these tasks help you achieve your goals so you might want to trim it down to a manageable level. Truth is, you need to be able to produce a sufficient and significant amount of output on your tasks, so much that will make you qualify as a productive individual. So trim those things down, and figure out what you can work with, after all, we are not machines eh?
Dedicate a sufficient amount of time on each task per day and stick with it. This is where self discipline comes to play. If you are not disciplined enough, you may end up underallocating time to a task - probably because you don't enjoy doing it - or overallocating. Self discipline is vital and will definitely help you with this stage. When you work on your tasks, just enough to classify as progress, then you can say that at least it is working. Just like computers with Multitasking ability, they have to distribute the available RAM across different sections, just enough to serve them and make them run smoothly.
Even though, I believe if you can avoid multitasking, you should but then it can also help you with time management and if you are disciplined enough, and you have figured out your capacity, you may be just as productive as someone who is linearly focused.