Founder depression is extremely common: https://blog.asmartbear.com/startup-identity-selling-sadness...
If success is the only thing that drives you it's inevitable that you'll be depressed when you become successful because you've just lost the one thing that keeps you going.
It's my dream that consumes me and keeps me happy more than anything else. But I too sometimes feel that "what happens after it's done" and it is not everything I imagine it to be. What if the bag of dreams we're all chasing is just empty like this Alan watts video(1).
Call it denial but I just try not to think about it a lot as I don't know what the future holds and it's pointless to speculate right now as every person is different. But reading your comment did scare me a little tbh!
(1) https://www.youtube.com/watch?v=ERbvKrH-GC4