There are different levels of it. Superficially, career success, relationship success, etc. being in situations that are enjoyable and self-sufficient as opposed to miserable and dependent.
Less superficially, there has always been a part of me that is ambitious. I was constantly told to be and expected to achieve something while I was a kid and was growing up so it's not surprising. Transition presented almost no fear for me, it was obviously a fair challenge not without risk and with high stakes, so I was more than up for it. I look back on the process so far and think of how easy it's been, psychologically, for me. Now transition is more than halfway done, I'm setting my sights on other things that need fixing or addressing, and ready to start the next half of life over. If I can do that I won't consider the first half a failure. I would consider it to have been boot camp.
Even less superficially I'd consider my life to have been successful if I was able to master all the issues and problems I've had (there have been many) and to feel ultimately content physically, mentally, philosophically. I'm already there philosophically - there was once a time when I couldn't stand myself or my outlook which is long in the past. There are still details to learn about people and experiences to be had. And physically I'm improving with time and transition.
For some people success would be having a kids, family, steady job they like, good marriage, retirement etc. Mine is a lot more focused on eliminating weaknesses and challenges. I don't know why, but maybe these things have to be done first before any of those. As well as moving away from everything I have discovered to be bad and unhealthy for me. I don't seem to do things by halves, it's all or nothing, and I want a lot of things that are terrifying to many people. I would like to up sticks and move abroad permanently... I would like to expand a serious career in the arts which is risky and fairly high stakes stuff if you put your all into it. I've begun a new life as 'someone else'. These are things not many people I know would even dream of doing.
If I could do all of these things from my humble beginnings and know that I did not chicken out I won't consider myself a failure. I suppose for it to have all been a success I would have to have a point where I would have reached the top and can look down on the climb with a sense of satisfaction. It's equally about the journey as well. Enjoying every moment for what it is, even the pain.
In short, being king of your own world and life, and knowing you are.