There are different definitions of maturity. For me, I believe that a sign of maturity is taking responsibility for your own part in what happens. I talked a little bit about this here.
So much depends upon our attitude. It's easy to get caught up in feeling resentful. In feeling like life owes us something. In blaming everyone else for problems.
At times there are legitimate concerns that need to be met. Legitimate wrongs that have occurred and that need to be addressed.
In my opinion, part of being a mature adult is recognizing just how much we are responsible for where we are at, by our own choices. I chose to go to a small liberal arts college. I chose to major in history. I chose to settle in the current place that I live.
No one held my feet to the fire. I might not have had all the information I would have liked when I made those choices, but I have to stand by them.
So there was a lot in my life that I haven't had control over, but there are parts of my life that I have had control over. I need to acknowledge my part in the grand scheme of things. And, just as important, that people are doing the best they can. Sometimes they're not, but often they are. Acknowledging that just as their hands are sometimes tied, mine are as well.
This last weekend, there was a system upgrade at my company. They asked for volunteers to check out the upgrade. I volunteered (for various reasons). Things are working well for me. Other people's systems are not working well. I overheard someone kvetching about the amount of testing that happened. So, granted, they should have tested more. But, you also had the opportunity to volunteer, and you chose not to.
Sometimes people just like to vent and complain. I certainly do!
But there comes a point where you have to let some of that go. My life is an amazing gift - every day is a new day for me. I can choose my attitude to whatever hurdles I might face. When I focus on how doors were closed for me, I can't focus on all things I'm grateful for, on all the opportunities I do have.