This is actually not all that uncommon a problem, although the fix for the problem is not all that well discussed. Sounds like this is a good time to talk about it.
Basically, you need to transition from a hobiest mind-set to a professional mind-set. That is to say, do not give yourself the CHOICE to stop working on one project and start on another one. Force yourself to just write on the project you are currently working on. Make it as much of an obligation as school work or a job.
After you get over this road-block, it will change the way your mind works and you will be more capable of sticking with a project. And also, although you have to force yourself at the start, this will not actually harm your enjoyment of writing in the long run. In fact, you will come to enjoy it more as you become more immersed in your worlds. You just have to bully yourself a little in order to get over that hump before you can start to experience the true joy of writing.
One more thing on this particular subject, do not become reliant on when the "muse" strikes you. If you insist upon waiting until the inspiration strikes you, you will never write a single thing and your writing will be unreliable. You need to completely break your dependence on the concept of what makes you feel "inspired," and just develop good work and writing habits. Make it a procedural thing rather than a chaotic random matter of when you are feeling inspired.
You can always write and let it sit if you feel unsatisfied with what you've written then come back to edit it the next day. 24 hours separation from your work is about right when it comes to being able to figure out what was wrong with the work you did before and see things from a new perspective. Do not wait longer or else you will have the same issue as "waiting for the muse."
Another thing I would suggest, learn touch-typing if you haven't already. The speed difference in the rate at which you can transfer your thoughts into words on a page will help with your ability to simply get lost in the process and keep going.
Also, one more personalized tip. I checked out your stories. The chapters are all WAY too short. You should be aiming for an average chapter length somewhere between 1500 and 2500 words. Your current chapter lengths are between 500 and 1000. There are several reasons why it is just better to go for the longer chapter lengths around the numbers I just mentioned. Those are the numbers that readers prefer to see. It is the nice happy medium between long enough to satisfy their brains but not so long that it makes their brain start to get tired.