Someone who didn't try to make themselves happy, and didn't try to make the people they loved happy. Those are the only failures I can see.
My cousin is at his happiest travelling and windsurfing. So he does what he needs to to spend as much of his life as he can windsurfing. That involves mostly working low paid jobs all winter and living in his van so that come summer he can up sticks and bugger off to Spain and spend 5 months of the year windsurfing in the sun. He's well aware that his lifestyle isn't particularly conducive to a wife or kids, so he's never had them. He'll die, having never achieved anything of particular note, with very few people missing him because he never keeps in touch with people long term. He's not a failure though, he took his life and did what he wanted with it, what made him happy.
My Dad on the other hand was probably quite successful in a lot of peoples eyes. He came out of school with barely an O-level to his name, but then found out he's an absolutely fantastic businessman and made a fuckton of money. Enough to retire in his late 40's. Coincidentally, he also loves windsurfing. So instead of retiring, he decided to quit the job he was good at but hated, and go do something he loved, and teach windsurfing to holidaymakers in a variety of nice resorts. Which if he'd been alone, would have been fantastic. But he wasn't, he had two teenage kids who very much could have done with their Dad around. Oh, and our Mum, who at the time had breast cancer and was going through Radiotherapy and chemo. He tried to make himself happy, but failed at the second part, making the people he loved happy. So he's a failure in my eyes.