Seek translation and understand sentences.

The grammatical structure analysis of the last sentence made by several people upstairs is complete enough. The only thing I want to add is how to understand this sentence:

Their fear-even before my father's letter-was disappointment.

Their worries are exactly the same as before I received (or read) my father's letter, but I don't want to disappoint people.

Let who down? Obviously let the parents down.

The following is the translation of the whole paragraph:

It is still far from 12, and I realize that he will be proud when I make any progress. However, I recently came to Bellevue, and he wanted me to take the next step: to be proud of others and believe in others. Tt, it's time to start writing letters to my children. Our children look at us with the same unanswered questions as us. Our children will not flinch for fear of failure. They are just afraid to let us down. They are just afraid to let us down. They are not worried about being disappointed. Their fear-even before my father's letter-was disappointment.

I have been 12 for a long time, and I believe he will be proud of my progress. But recently, I believe more and more that he will expect me to move towards the next goal: to trust others and be proud of others. I should start writing letters to my children. Our children looked at us with the same doubts as we did. They don't flinch for fear of failure. They are just worried that our poor performance will make us ashamed, and they are not so worried about their own gains and losses. Their fears are exactly the same as before I read my father's letter, but I don't want to disappoint people.

Note: At the time of writing this passage, the author's father died of natural causes. It is obvious from his voice that he is proud and he wants me to move on. Personally, I guess this letter left by his father is the last relic, and his father died unfortunately at the age of 12.

I hope it can bring some help to your study and understanding.