The Buddhist have this concept as part of their teachings - impermanence - and while I am nowhere near qualified to talk about Buddhism, I will discuss the concept and how I apply it to my life because if it's helpful for me it may be helpful for you as well.
My over-simplified understanding is this: nothing lasts forever. Good, bad or indifferent, it won't last forever. The only thing that's guaranteed is change. We spend a lot of energy trying to get away from that which causes us pain and trying to get to that which makes us happy and yet it's all continuously changing. Nothing lasts forever. So when I struggle, when things are tough, when I'm in a slump, I remind myself of this concept and just work on accepting whatever is wrong because I know that it won't last forever.
When things are good, impermanence reminds me that I should be fully in the moment enjoying all that is good because that also won't last forever. That's not to say it will turn bad, this isn't intended to be a pessimistic point of view, but rather that it will change. Thinking that you always have tomorrow to enjoy the things that make you happy is a mistake.
Regardless of what might be happening, I work towards acceptance. Accepting what is rather than struggling and pushing to get away from the pain or to obtain the happiness, brings peace and tranquility. These Buddhist concepts are so simple yet so complex. That's what I like most about the philosophies, they get you thinking.
I hope that with this short post I can plant that seed of thought and that by keeping the concept of impermanence in mind, you're able to cope better as well.