Steve Jobs was outsed from a company he founded. Did he feal defeated? No! He went ahead and founded NeXT, a computer company headquartered in Redwood City, California, that developed and manufactured a series of computer workstations. The company was later bought by Apple and the innovative products can be found in Mac OS X.
It is worthy to note that Jobs was called back to Apple and made a huge comeback in growing the company, lauching the iPod, iPhone and iPad range of products that became huge success.
It's never late to start something new, you're never too old. It's over only at death. If you're feeling despondent at this moment, gather strenght from this tiny story of success, start something, make it better. Failure is a process that toughens you up for greater success.
I have failed myself many times, but never see myself as a failure, rather I have kept learning and getting better. I feel tired too, no one is imune against it, but I never tire out. Success is celebrated when achieved, but there are lot's of work, sweat and trying times that went into it.
Get up, dream again and go to work on your dream. Is it a startup? Keep working and learning.
Find people who will support you and give you strenght in the dark days. (For startups, look for innovation hubs to partner with)
Most of all, you owe it to yourself to never give up. Remember Steve Jobs.