I've had what feels like an insanely successful year in a lot of ways. I've sold more than 20 stories this year, I've gotten very nice feedback on some of the stories which have been published, I've made it 2/3 of the way to full SFWA membership, etc.
However, there are still lots and lots (and lots) of milestones which I haven't even come particularly close to reaching. So, looked at against the backdrop of those other milestones, it would be easy for me to say (were I so inclined) "Well, I really haven't accomplished much yet." And, looking at it purely objectively, there's truth to that. If I stopped writing today and published nothing besides what's already in the pipeline then twenty years from now I'd almost certainly have left functionally no mark on the word of speculative fiction literature.
So how do I look at this? Have I had wild success or am I still down on the lower rungs?
I believe the answer is that both are true. Writing is an interesting game as far as "success" goes. Unless your name is Stephen King or J. K. Rowling or one of those other Really Really Big Names, there's almost always another star to aim for. (And still, you could certainly argue that both of those authors, even after reaching enormous financial success and fame, felt they had some other targets to aim for by reaching outside of the genres which brought them their initial rewards.)
Ultimately, each writer will have their own idea of what constitutes "success." Like I suggested above, for me, that would be doing something that made it reasonable to think that ten or twenty or thirty years after I'm gone from the Earth that there would be people out there who found something of value in my body of work. (I think that the jury is out on whether this is going to be something which is easier or harder in the digital-fiction era. I suspect that it will be easier, but I also wonder how institutional libraries are dealing with electronic-only publications.)
If you define "success" as only being those very ambitious goals, it would be easy to burn out through the early years of rejections and smaller victories. Conversely, I think it's good to stay hungry and to want to reach for the next "level" even as you're making good progress. I would encourage writers to be willing to look for and be excited about successes along the way to whatever their ultimate marker of success is.
What does success look like in your world? Do you celebrate the steps along the way or are you waiting for the "final level" whatever that may be for you?