Here is my response:
I do think about death more now, because 40 sounds like I'm getting close to halfway, and because we've been reading the Bible which has a lot to say about death and about how to live.
On the same day that you emailed, a thread on a running discussion forum (where people often discuss topics other than running) was posted with the topic "Guys between the ages of 45 and 55, do you think about death or does it seem far away?" I agree with this reply:
As the imminence of death inches closer I think less on worldly, material matters and concerns and more on things that are everlasting. Time with family becomes more important while having the latest gadget or a fancier car or the like become less appealing. Developing greater patience, kindness, empathy, etc., are more valuable than worldly status and recognition. Truly, the closer that I inch towards the eventual end of mortality, the more I recognize what a blessing and gift that each and every day of life is.
That's well said, right? They agree with your feeling that what you want has "nothing to do with money," when they say they "think less on worldly, material matters and concerns."
Similarly, the Bible says:
And do not be conformed to this world, but be transformed by the renewing of your mind, that you may prove what is that good and acceptable and perfect will of God. (Romans 12:2)
"Conformed to this world" means living the way society says you should live - desiring all the products we see in advertising, idolizing and mimicking the people that we see in entertainment, and following all the traditions that society has created.
I'm also not planning to have kids, but I don't feel the need to leave anyone or anything behind after I die. After we die I don't think we'll be aware of any family that we left behind. I don't think we could feel that we're continuing to live through our children, so I'm not unsatisfied by not having children.
You said, "I feel I'm nothing without accomplish[ing] anything." Do you mean you feel you are nothing because you haven't accomplished anything? I understand that to be happy we have to have some sense of accomplishment, but I don't think it's necessary to have a big impact. More important is that we feel that our work at least helps people in some way, and doesn't harm people. Personally, I find that to be happy, I need to have others tell me that I did a good job - if you're doing work like teaching Japanese, then hopefully you get some positive feedback from your students, and if so, you should feel like that's a great accomplishment and worthwhile work.
Another point the poster above made that I agree with wholeheartedly is "Developing greater patience, kindness, empathy, etc., are more valuable than worldly status and recognition." This is exactly what I started to feel when Andrew died. It changed me and made me more patient and kind with my students, because I realized life is short; there's no point in being cold with people.
The poster concluded that they increasingly "recognize what a blessing and gift that each and every day of life is." That's exactly how we should feel. Part of that is cherishing our relationships, but I think it also includes appreciating the beauty of nature. I love the sakura photos you sent! Even though I mentioned above that societies have problematic aspects, I think Japanese society does a great job promoting awareness of the beauty of the seasons. I have taken that away from my time there and now I love to see the flowering trees and colorful autumn displays. We should feel satisfied by living in such a beautiful world.
Finally, I mentioned above that the Bible says how we should live. Specifically, it describes all the sins that we should not commit, and the fact is that nobody can live without sinning. I've seen enough evidence to believe the Bible is true, so I believe it when it says we will be judged after we die. Since we all sin, we would all be judged harshly, but if we repent (feel that what we've done is wrong) and believe that Jesus, the only person who lived without sinning, died for us, then his death is accepted as payment for our sin, and we can be judged as if we did not sin.
For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life. (John 3:16)