I saw a squirrel humping a toad.

No, not really.  That idea just randomly popped into my head.  I wonder if it's ever happened.  The offspring would be a large hairy toad with a snout, ears, and bushy tail, don't you think?

Why can't my imagination run wild like this with one of my story ideas?  It used to.  I wonder if I have too many ideas up in my head.  They do vie for attention all the time; it's frustrating.  I end up asking everyone in my head to shut up.  I need to concentrate on one story and stick with it.  The issue is I don't know where to begin.  I have no inspiration for any of the stories I think about writing.  But--

This is what Cassandra Clare says on the subject of inspiration:

How do I get inspired and stay inspired to finish my work? How do I get motivated? How do I keep from getting distracted? 

We should be taught not to wait for inspiration to start a thing. Action always generates inspiration. Inspiration seldom generates action. —Frank Tibolt 

Inspiration is wonderful when it happens, but the writer must develop an approach for the rest of the time… The wait is simply too long. —Leonard Bernstein 

I am not the person to ask about inspiration. Mostly, I don’t believe in it — not in the popular conception of inspiration, where a lightbulb goes off in your head and suddenly you are inspired and your fingers start flying over the keyboard. Sure, that can happen, but as Bernstein says, you can’t rely on that happening. Writing is hard work, work that relies on learning and applying a varied set of skills, and finding out what those skills are, learning and practicing them, is always better than waiting around for inspiration. To quote Kristi Holl’s Writer’s First Aid: 

Writers who wait for inspiration before they decide to write are generally known as hobbyists. Working writers-those actively writing and growing in their craft-must write whether the muse is “in” or not.

Not many professional writers believe in inspiration, and the more I try to write a book, the more and more and more I don't believe in it either.  Writing is hard work, like any other job out there.  I have so many excuses about why I can't write... I'm too tired from work.  I have writer's block.  I can't get enough alone time.  Blah blah blah.

Any suggestions to make me shut the hell up and get to it?