I love that innocent joy I use to have all the time as a child, that extreme form of peace and curiosity that seemed to arise from within me. Last month, I started experiencing that feeling again in like 10 or more years, but as soon as I tried to hang on to it, it disappeared. Just as you said.
I learned that in my case I get the most out of that happy feeling by not hanging on to it, but cherishing it for as long as it is there. I let the feeling roam free, as it will disappear when the time is there. I feel that all I can do is to be grateful that I am granted to experience that joy one more time, and make the best of it, rather than hanging on to it and worrying it will disappear. Worrying makes it disappear in my case, since worrying will take out that innocent part that makes it so child-like.
Maybe that one day you will find a door towards that feeling Jeannie, I hope so.