I am almost 50 and have only just realised that I have a really weird issue with this. If I attempt to even think about it, my head won't work!
As far as my lifestyle is concerned, I have done what I loved but been very lucky (creative degree then work in same field) but outside of this I have never once been able to commit to anything or carry things through.
As a child I was passionate to join the brownies, then the girl guides, started then quit both. I did ballet, rode horses, played a couple of instruments, did tennis - and whilst I was good at most of them, I just quit.
I wasn't shy or clumsy, and didn't have any issues, I just recall feeling a variation of either boredom, anti climax or a simple 'nope'.
As a teen I did this with hobbies; drama club, dancing, whatever.
As a young adult I started then quickly left quite a few jobs before and during uni.
I now see that my parents were not strict enough, perhaps. They always just said 'ok, don't worry!' and sorted things out. I wasn't spoiled or pampered, but I was never forced to commit to things or to work for anything that didn't come naturally to me. I was lucky that I did well academically, but two exams were missed as I didn't want to sit them 
. I was soundly told off for this as my parents had to pay, and I did resit them the following year. It is sheer luck and a fair bit of privilege that I had those choices.
As for why my folks were this way, I think it may have something to do with them losing my brother before I was born when he was still a baby. This horrific lack of decisiveness and commitment has not affected my older sister.
How can I learn to change this now? I think that this aspect of my personality is outgrown and not doing me any favours at all. I am at the end of my long term relationship and starting afresh, so some connection with focus and goals would serve me well.
I never married and couldn't have children, although I have been happy with my lot. My long term relationship lasted 20 years so I am comfortable committing to people. It's just that if I try to think of a new career or things to do in the future, any kind of planning, my head just can't grasp it.
How to develop this capability? If I was never pushed to develop it, is it even possible? My adult life has not been as privileged and cushioned, but I have managed to just go with the flow, which is great - to a point. I just don't know how to even want a goal let alone work for one. I certainly have a dream but have no idea how to really achieve it. I am not remotely ambitious and would be happy to just live a peaceful life with meditation and art, but I fear dithering about like this is too unbalanced and would like some more backbone.
Sorry it's so long. Any tips appreciated!