These musings borrow heavily from the work of Irvin D. Yalom.
There seem to be four problems, four things with which we struggle, that we can't do anything about. That is, there seem to be four conditions to which every unsolvable problem can be reduced. Their examination is useful because the task of human existence can be understood as a process of coming to terms with them.
There are different ways to put them, but I use the mnemonic "mipi" to remember them myself. They are meaninglessness, isolation, powerfulness/powerlessness, and impermanence.
Meaninglessness refers to the fact that there is no inherent meaning expressed for existence. That is, there's no declaration of what this is all about which is so obvious and clear to us all as to explain why we live. We have to find that meaning for ourselves and there's no way to confer the validity of what one person believes to another. I can't convince you of the meaning of life, you have to realize it. We may agree, but that's a choice we're making, not because of a self-evident Truth.
Isolation refers to the fact that we are all ultimately alone. No matter how close one becomes to another there is still the persistent reality that we remain ultimately alone. There is no bond between people that can't be lost, and any bond that exists still fails to close some small bit of the gap that keeps us aware that we are separate beings.
Powerlessness and Powerfulness are two sides of the same coin. On the one hand, we are free to do whatever we want to do with our lives. That sounds like a wonderfully liberating thing, and it is, but it also carries the responsibility. That is, if something is not working for us in our lives there's really no one else to blame for that, ultimately. No matter how put-upon one feels, the fact is that you can do what you want to do.
The flip side of this refers to other people. That is, they can do what they want to do, whether or not it is right, whether or not it is a good idea, whether or not it is going to help or hurt them. I used to experience this as the awareness that there is no "relationship court," that is, no where I could go and make my very cogent and powerful argument that another person had made the wrong decision by leaving me and getting married to someone else. I know she was wrong, but I couldn't do anything about it. It didn't matter that I was right, it didn't matter that she was wrong. I couldn't do anything about it.
Impermanence is the big one. This is the central teaching of my religious tradition. Things change. Anything that lives is going to die. You and everyone you love is going to die. Every relationship you have is going to end. Anything that exists will cease to exist. Everything, no exceptions. Nothing persists. Nothing.
So, now consider engaging in what I always find to be a helpful exercise. Think of each of the problems in your life. Start to examine them and figure out which of (or which combination of) the four problems this dilemma reduces to. That tells you what it is about that particular situation that you have to accept. You can't do anything about that fundamental fact, so your capacity for action is guided by this realization, you don't waste time and energy trying to change something that can't be changed.
Recovering alcoholics have a notion they call the Serenity prayer, it has various exact texts, but they all go like this: "Help me change things I can, accept the things I can't, and give me the wisdom to tell the difference." The four problems have given me a structure within which to do that work. Give it a try.