Amazingly, with much of your recent post I agree, even though I do believe in God, that Jesus is the Son of God, begotten before all ages, and that after his death, God raised Him from the dead, and that He is Lord of my life.
You wrote:My assumption is that I'm truly mortal.
I, too, believe I am truly mortal. Indeed, the apostle Paul wrote that God alone has immortality (1 Tim 6:16 ESV)
My mind is an emergent property of my brain and it did not exist before my brain developed.
This is my belief also.
Likewise, my mind will be destroyed with my brain when I die.
Agreed. Even the apostle Paul indicated that unless there is a resurrection of the dead there's nothing more. He wrote:
“...If the dead are not raised,'Let us eat and drink, for tomorrow we die.'”(1 Corinthians 15:32)
This is unfortunate and pretty disturbing to think about.
Here is where I begin to disagree. Though I believe that when I die, I'm dead, and no longer exist (including my mind), as a Christian, I have the fond expectation of being raised from the dead, just as Jesus was raised. The writer of Revelation called Him "the firstborn from the dead." However, I don't believe that I will be raised until Jesus returns. That could happen in ten years or in ten thousand years or more.
...but it doesn't mean I have no sense of purpose or morality while I'm here.
I fully agree that an atheist can have a sense of purpose and also recognizes moral imperatives—to benefit other people, and to refrain from doing them harm.
To the contrary, I get a sense of purpose from the idea that this time is all I have.
You may have a sense of purpose for this reason, but I must disagree that this is all the time you have. Although this may be all the time there is for you now, the time will come when God will raise you to life again.
If I don't like how things are, I personally have to change them.
That could be hurtful. What if the things you like would result in greater harm to people?
But even supposing you like only the things that benefit people, what if you are unable to change the harmful things that people do to one another?
Another thing I would like to ask you. Are you fully satisfied with your life? Or do you feel that there is something lacking—and that you can't quite figure out what it is?