Ideas, it seems to me, are like stray cats. First of all, they’re everywhere, even when you can’t see them. Sometimes, especially when you can’t see them. Sometimes you see them, try to approach them, and they dart away. Feral, you know. Other times, you see one, it allows you to pick it up, it nestles in your arms, and is thrilled to be found. But, occasionally, one shows up in your yard. You didn’t notice it arriving, just, suddenly, there it is. It may run away if you try to touch it, but after you retreat, it may stay and hang around. Kind of always there in the background. If you leave out a bit of food, it may wait till you’re not looking, then take a nibble. If it starts to eat regularly, word gets around, and other stray cats are likely to show up. If you begin to nurture them, rather than ignore them, they stick around. Eventually, one or more may become tame enough for you to bring it inside, get rid of the fleas and other flaws, and teach it good behavior, like using the litter box. Soon you have a well-nourished, healthy, flourishing cat. Then, like ideas that you have developed into a healthy piece of writing, you try to find it a home. When that happens, you’ve made room for the next cat–or rather idea.
Certainly you can foster multiple cats, or multiple ideas at a time. But don’t worry about letting any of them go, if they are not meant to be domesticated by you. There’s always plenty more out there, just waiting for the right house, the right yard, the right moment, and especially, the right person.