Success for me is the feeling of being content. Its simple, I'm never content until and unless I've either spent my entire energy on some task and failed or until I've succeeded in the task.
Either way I feel successful, and mind you I'm very proud of the times when my efforts don't bear the perceived fruits, because I truly believe the bigger picture is something none of us can see!
I see a lot of people saying success is when their parents smile at them and/or are proud of them. But parents are human beings and can be fooled into feeling proud of you. The only person you can't fool is yourself so be true to yourself and give your all!
Correspondingly, if I'm not content I become restless and uneasy which is easily the worst failure one can experience!
Contentment is for sure a difficult mindset to develop. But it helps to put things in perspective.