To the homesick missionary:
Your mission is full of your highest of highs and lowest of lows and it will be like that up and down, up and down. Don't dwell on those lows. Focus on the highs. When weeks like that happen, it will take a couple of days to get over it (trust me, I know), but don't let it turn into a couple of weeks to get over it.
Homesickness will come back every now and again and haunt you, but look under diligence in chapter 6 of Preach My Gospel (p120, I think?) and read the quote by Pres. Benson. That's how you can overcome it every time.
The good 'ol times were great, weren't they? But, guess what? Your life has changed ever since you sent in your acceptance to your call to this mission. Do you think James and John knew that their lives were going to change when they left their fishing nets to follow Christ? Probably not. But you can know this now: your life has changed the moment you stepped onto the mission field. And it's changing for you to be closer and closer to Christ.
After Christ was resurrected, Peter, James, and John went back to their fishing nets. They went back to their old life. That's when Christ appeared again in His resurrected body and asked Peter, "Do you love me?" three times. Peter probably got a little annoyed after the 3rd time, but Christ was actually telling him a LOT more than just to feed His sheep. He was basically saying, don't go back. Don't go back to those thoughts, those feelings of nostalgia, or to a yearning for your pre-mission life. Focus on the now. Your life has changed and will be changed forever. Don't look back. Our Father has great things in store for you.
I'm not telling you to abandon your memories of home. Keep them close to your heart! But your mind and heart can only focus on one thing at a time and that one thing now is the Lord's work #consecration Think of all of the good times you're creating right now!
The only thing that can hold anyone back are the very thoughts that Satan puts into your mind. He wants to stop you. Don't let him.
"Wherefore, may God raise you from death by the power of the resurrection, and also from everlasting death by the power of the atonement, that ye may be received into the eternal kingdom of God, that ye may praise him through grace divine. Amen." (2 Nephi 10:25).