I've heard stories of successful people failing in business and never coming back and even dying feeling like a failure.
What is the best way to avoid that? How to come back stronger?
Well, it's the business that failed, not the entrepreneur. There are always lessons that you take away from the experience - something that people who didn't try will ever have.
This knowledge is both rare and valuable. It can be shared with others, or it can be used to generate wealth in the future. I'd suggest you make en inventory of items that you learned from the experience. It could be something tactical (e.g., "learned how to run a proper AdWords campaign; learned how to build a professional website") or more strategic. In either case, you're going to be better at starting up the next time around.
Exercise 5-6 days per week, 20-40 mins per day. Aerobic, weights, whatever. Don't over-do it; exercise is not punishment. However, it is vital to keeping your mind healthy. Scientific fact.
Get enough sleep.
Eat well.
Brainstorm ideas. Have friends (potential new partners?) help you. Any idea is worth playing with. Develop a list and keep adding to it.
Brainstorm needs. What do people need? What problems do people have that they may pay to get rid of?
Go to Hacker meetups.
Understand your mind and how it works. Mindsight by Dan Siegel is excellent.