The "problem" with yesterday's perfect practice session... that you will practice again today, and you expect it to be that wonderful.  I remember having such a session as a student in college.  I began with long-tones, leading into a very slow free improvisation, leading to more complex free playing into a tune of some kind.  My mind was buzzing and I moved onto my technical exercises, etc., eventually moving back to a free improvisation to end it.  I felt amazing and was so excited to share what had happened with my teacher the next day.  I felt like I had licked this practicing thing!
It's hard for me to describe the look on his face when I described the session, but it really disappointed me.  It was sort of all knowing.  It's not that he wasn't happy for me, but there was a look of warning in his face, too.  He said something like, "They're not all going to be like that."  This really bummed me out at the time because I thought I was following his instructions and had really gotten somewhere.  I thought it would be all gravy from here on out. 
Twenty years later I totally get it.  Of course we should enjoy those moments - why else do we do this, right?  Just know that they won't all be like that, and that is OK, too.  What a great lesson the pursuit of a musical instrument teaches us!  Throughout your artistic path there will be an ebb and flow to your abilities, practice habits, joy, and inspiration, not to mention the frequency of performances.  You'll never finally "get it" and be fine from now on.  How boring would that be anyway? 
I try to remind students of this on occasion, to take the pressure off that you will reach a point when it's all going to be easy.  Many things will get way easier and that feels awesome!  It's just that you will always be reaching a little higher at the same time.  I remember what it felt like for "Stella by Starlight" to be a total mystery to me.  I now know what it's like to feel totally free and comfortable on that tune (at times - with the right people in the right circumstances...).  But there are so many things I still can't do. 
Ego check: Lee Konitz said that he's been playing "All The Things You Are" for 50 years and is still trying to get it right! 
So there you have it.  That is the mark of someone who has fun, stays engaged, always pushes and tries to discover something new.  Knowing that there will be good days and bad along the way.