Editing, editing, and more....yes, editing

Editing is such a huge part of writing. It took me three months to write my first novel, but it has taken three years to edit (partially because of LIFE, partially because it takes so darn long). Now, with awesome feedback I've received, I'm editing some more.
I am currently reading through my cousin's book. She wrote a YA science fiction novel. It is so much easier to look at someone else's work and say, "Hey, change this. Delete that," than your own. YOU know what you want to say. YOU know what feeling you want to come across. But the ultimate test is...does everyone else understand it, too?
I feel like I could edit forever and never have a complete, finished work that doesn't need some tweaking. I guess at some point you just need to throw in the towel and say, "It's good enough."