What about the general impression: is not my essay too easy? Thanks in advance)
Ok... let's take your prompt again;
Most experiences in our life that seemed difficult at the time become valuable lessons for the future
You started the essay saying;
Human life reminds me endless education that commences at infancy and goes on to death. Beyond doubts, this "studying" is accompanied by happiness and sadness, that are both important and necessary, although some of us wonder, why troubles are so valuable.
... well, when writing the introduction, your primary objective should be to introduce the topic/prompt clearly. In that case you can get lots of help from the prompt but need to rephrase it without copying one to one. Here you have attempted to get an entrance, but it sounds too complicated to the reader. Until I saw your topic, I couldn't catch the idea clearly. I feel you have some idea about the essay structure.