I go through that problem a lot as well, sometimes I am enrolled into 5 or 6 MOOCS at the same time, just because I find them all interesting. The way I do to get over this and finish some of it is by watching a couple or more lectures from all courses and decide which one I want to be pursing til the end. As many people have noted here, you have to choose the things that you want to accomplish and is most important for you at this point of your life and stick with it. That does not mean you cannot go and have a look at something from other topic that you are interested in, just that you need to concentrate and focus on some things only in order to really get a grasp of it.