Living in the past:
From my experience the people who I know that are least happy with their life seem to be unhappy for one main reason, they can't let go of the past but more specifically they can't let go of the image they had of their future as a child.
Childhood dreams:
Everyone as a child has an image in their head depicting how they want their life to turn out and people try so hard to hold into that image they can end up becoming frustrated and bitter toward life when it doesn't come to fruition. The reality is we are constantly changing so what we wanted as a child will often not be what we want as adults so there is no reason to follow dreams that don't exist anymore. Also as a child the views we have of the world are warped so if someone wants to be a lawyer as a child they don't think of all the hard work that comes with that and the amount of time you have to spend working day in day out and that type of lifestyle isn't for everyone so if you don't enjoy it why hold into it, because as a child its what you wanted?
Trying to stay true to your childhood dreams is noble but also insanity in my mind. However if you have a dream job as a child and still want those same things as an adult and you are willing to put in the time then I would say you would be crazy to not do that thing.
There is know reason to try and stay true to who you were as a child, it seems hard for some people to let go of who they were as a child, I think because as a child you hold so much innocents and hope but people also seem to neglect the fact that as children we know nothing of the world. Staying true to a past version of yourself will only bring harm to yourself and those around you, it is a lot easier to be happy when you stay true to who you currently are.
Personally as a child I wanted to be an inventor, entrepreneur, architect, painter, vet and writer all at the same time which just isn't a realistic goal and if I tried to stay true to those goals I would be very stressed and unhappy because that isn't what I want now, it was a lovely idea with good intentions but it's not what I want not, I also wanted to marry and have a family by then time I was 22-25 and now I couldn't imagine anything worse.
Glory days:
It similar to when you see someone living in the 'glory days' of their youth, which many people fall into the trap of doing. There are times when it can be hard for people to not live in the 'glory days' especially for people who were on track to be athletes and got stopped from doing so via injury or similar circumstance. When people never get over that and spend so much time living in the past and dreaming of what could have been they make their life into a mess of negativity, bitterness and are unproductive an unmotivated.
Not making a decision and feeling powerless is a big factor in this, when people feel something is forcefully taken away from them they can become bitter and it can be hard to get past and inevitable it will take time, but it has to come to a time where you just get over it and move on, there is nothing good that will come to anyone refusing to move on.
When we have obstacles in our path we can let that obstacle stop us from moving forward and let it define us or we can move it out the way and keep on going and let our actions define us. When people refuse to move on from something they really bring sadness upon themselves and push everyone away from them and honestly end up being quite pathetic. If something stops you from doing the career you wanted or stops you from having something you wanted you have to either work harder to realize that goal or just move on from that thing and find something else and because we are always changing it is a lot easier to do than people thing.
So whatever it be for whatever reason it may be people who hold onto the past always end up the same way, the person that can't let go ends up unhappy and pushes people away because they can't help but project their emotions and people that move on from the past will be happier and likely more successful. I also don' think it is as hard to let go of the past as people think you just have to accept it, it happened you can't do anything now so letting go and moving on should be the only option.
This is not to say people should give up on their dreams if they are possible but you that people should really think about what they want from life currently and go from there rather than sticking to what they wanted years previously.