There is this one wall that I always practice on, it is about 10' tall. Recently, for the life of me, I can barely ever reach the top.
How many different paths to the top have your tried to follow?
I'm very sure if you spend a whole training session brainstorming and troubleshooting, you may find up to two or three other paths to the top. Furthermore, in adapting to different approaches you may find that in time the first approach will once again become easy.
Much of "explorative" Parkour, in my opinion, involves critical use of problem solving skills. Part of problem solving involves taking you mind off the problem and returning to it at a later point in time. Often by the time you return the problem is no longer a 'problem'.
What this translates to physically:
Sometimes we meet obstacles that just, at some point or another, defy our attempts at surpassing them. I believe that when this happens you should leave that particular obstacle for a time and look for an easier obstacles to solve. Then when that former obstacle is no longer your primary concern, probably because you have encountered a new challenge, go back to it and try it again. You'll be surprised how much easier it seems by then.
I can't kong a picnic table anymore
Can you run on top of it
Can you roll over it
Can you slide across it
I'll admit, practicing predefined moves is great for building skill and ability. Theres nothing wrong with that, in my opinion. In relation to that, I would advise you to trying the same approach from different angles and slowly progress till your can do it straight horizontal. Or find other easier "kongs" and drill them until the picnic table becomes natural.
What I'm actually trying to address here, is that regression is often choice or forbearance there of. There is more than one way for you to progress in your chosen 'parkour'. Don't restrict yourself to a set way of moving at the expense of being unable to move in your own environment.
I believe that it is important to practice your ability to adapt quickly to any situation. That way when you meet a situation requiring use of a common form of movement, you adaptability will aid you in surpassing it. (However, drilling is still just as important, just don't forget to compliment it with adapting)
I have a feeling if you try looking at your abilities from a different perspective, you may realize that you have not regressed (backed tracked in progress) at all. What typically happens in practicing any discipline is that just before you reach a plateau of ability you encounter a slight peak in performance. Once on the plateau this can become quite discouraging. Don't worry though, its part of a greater progression cycle and before you know it, if you focus on simple things, you will progress far past your former peak.
I wish you all the best, just keep with it.