Lively Challenge - WiT

Here's a lively challenge
With a different slant.
Pen a verse but never
Let it be a rant.
Twenty six great letters
In the alphabet are there.
Twenty five are plenty
Just use them with great care.
The missing letter's vital;
It's there, let us recall,
In frequent regularity,
Helps keep us in its thrall.
My brain is really struggling,
Might end up in free fall.
Admire this perfect letter,
It's there at every turn.
I'm really up against it,
My mind is all a churn.
I'll check what I have written
I'll check it with great care.
I think I have succeeded,
It really isn't there.
Hurray