The on and on of sequels...how many is enough?

I love to read book series.  I love to be able to go back to the people I met in the first story and get to see what's become of them.  The best series fully incorporate those beloved characters.  Dee Henderson does a great job of that with the O'Malleys.  As each book progresses, you get to see relationships evolve (though she isn't perfect, there are still characters that get completely short-shifted in some of those books).  

When I first started writing When Broken Sparrows Fly, series was not in my head.  However, once I settled on how it should end, I realized that there was more story I could tell.  What about Beau?  Does she find that perfect guy?  Of course, hers wasn't the next story I conceived that would go with Kat's story.  That honor belongs to Lucy.  So, as it was, I came up with the idea of creating a trilogy.  I even have names picked out for the next two--Under the Wings of Grace and Soaring on Hope--get the bird metaphor?  

Tragically, I haven't gotten very far.  I actually have about 8 chapters from the second one done. Lucy's story is off to a great start (and I think it's even kind of unique) plus we get more about Kat, Max, Beau, and Theo.  But, I've kind of stalled out on this project.

This is what makes me a little nervous about sequels...what if I can't finish?  What if it just dies out and leaves the first just kind of hanging there in the wind.  We want to know what happens? What if I am unable to deliver?  

I haven't even touched Lucy's story in a couple of years...when will I ever get back to it?  Since then I've come up with a couple of other stories that I prefer to tell.  And, don't even get me started on poor Beau...will she ever have a chance to shine in her own story?  Maybe, maybe not, since I've only gotten so far as a rough idea of what that might entail.  At this rate, I'll be sixty before I get to it! 

So...is a series a good idea?  I'm not sure.  I'll let you know.