Here's the harsh truth. Burnout is going to cost money and time to fix. It's not simple. It's likely your brain is saying that things aren't right in your world representation. The best long term solution is understanding yourself and learning to explain what your subconscious is telling you.
Maybe your startup is taking more time to develop than you thought. Maybe your anxious about the market impact, the usefulness, or something else related to your timeline. Or you might just be watching too much CNN. You have to identify the variables that are contributing to your state(disbelief in your own world model will do it).
I don't believe that the root cause of depression is always nihilistic, although existential crisis' do exist. That's more about the state of being - and not everyone who faces the truth of reality crumbles.