Good question by Claudia. You read about authors losing their creative abilities, and might read last books by authors that simply lack the "oomph" of earlier works. I think it is taken for granted that everyone has his peak and his inevitable decline, slow or fast. There are numerous authors whose habits (often alcoholism) guarantee this and others who simply succumb to old age - Graham Greene comes to mind. Why does this happen? On the one hand, old age steals just about every kind of energy, especially physical and, I would say, psychic creativity. It is a process of nature in all the creatures beyond the cellular level. From there, we go into a "winter" of sorts, the hibernation whose deepest sleep is death, to emerge in spring only God knows where.
The tree stops supporting leaves and so on - something we are all aware of and find in it, in contemplation, not despair but an odd kind of thrill, an inkling of how the universe is put together, along with us.
Less philosophically, you can look at the ups and downs of lots of professionals. Athletes who sprint top out at about 25, while marathoners can be at top form into the mid thirties. Poets are usually best (so it seems to me) in youth, while novelists usually peak in their 30's and 40's. Artists seem to go a bit further, perhaps into their 50's or 60's, but everyone has a peak. From an individual perspective I have probably passed my peak, but wasn't active as as creative writer until relatively recently - so I only know for sure that I feel more tired than I used to, which probably affects my work.
And let's be frank - we get no respect, and that depresses and depression isn't good for anything. I have noticed that you (we) are not taken seriously until you are published, and then again are not taken seriously until you make money from the publication. You can plop the same book onto someone's lap - if it has been successful commercially, he will read it and beg for your autograph, and if it is just a manuscript or self-published thing, he will sigh and smile and put it aside, hoping to never have to read it. I am not blaming the put-upon reader in this, mind you, but how we react to the reactions, which is: discouraged and, in that frame, depressed. I know that in my first three books I felt this great sense of accomplishment in the end and felt compelled to have others read it -and I also knew that it was just a matter of time before this 'genius' was realized by all and everyone. Now, I might tell my wife that I have finished another (to which she rolls her eyes) and am embarrassed to give it to a anyone, lest I am giving them an onerous chore. I have to think of the words of the sages, not least of them Jesus, who have all said more or less the same thing: do your work without reward. In Jesus's parable, this was taught to the person who would be generous or holy: if you proclaim it, the adulation of the crowd is your reward; if you do it from the heart without seeking reward, your reward will be in heaven.
As far as this pertains to books, well ... I guess you just do it because you feel it in you and want it to come out, no more or less like a song or a skip in the park. I don't know if you (us, 'one') gets a reward in heaven for this, but you do have your work, your "stash" and, more importantly for me, the process. It is this final thing, the process of writing, that makes it the most valuable. It is, as you and everyone who writes must know, in the writing that we discover things in ourselves, things that come out of hiding from some infinite space beyond. As for me, hell yeah, I'd like money - but I might have to be content with the results and the process. It's way better than nothing. And there is an advantage to being unsuccessful - when the creativity starts to ebb, who will care or even notice but you? So you can quietly come to terms with it and move on to whatever calls next. Think of those poor famous people who draw ridicule for their later poor attempts.
But discouragement? It is the way of the world - and there is a lesson in it, as always. Do not depend on the world because it always changes, and so will always disappoint in the end. Oh, yes, waaay easier said than done, but so obviously true.
So I'm glad you brought this up, Claudia. I needed a little comfort myself, and find it, as always, from the deep well of Truth as given from the truly wise that is constantly calling, even though I always manage to forget and turn away.