As for "not getting married the an idea", that really is just a warning about the nature of the business. You are, after all, in the ideas business, and you need to keep churning out new ideas. Unless the project you are working on is a real labour of love that you will continue to produce for yourself even after picking up a paying gig; put it away in a drawer. Again, you just never know, it might be that 6 months down the line you'll open the drawer, look at that project and it'll look good to you all over again.
So, what about putting these ideas into practise? Well, hat's a wee bit more difficult, especially when you can easily imagine yourself completing everything easily and on time. Well, you have to take stock, I'm afraid, you just have to, there is no avoiding it, even if it means admitting that you are only human. It is also a very valuable exercise because, in case you haven't noticed it is happening, you will soon find out if you have become an obsessive, slightly manic, gibbering wreck; which is, by the way, what you will become if you continue to try to work on everything all at the same time.
One project I have in the drawer at the moment is Mandrake Falls, which I hope that one day Dwight McPherson and I might finish together, along with a colourist (I hope). The plot, Dwight's, is still strong, the title is great, and the thing looks good, think (see below); but it is an idea, I think, that will profit from being in the drawer for maybe 3 months - which, coincidentally, is exactly the size of my backlog; although that may even be an underestimate.