The time to prepare for any holiday that promises to be stressful is long before it ever happens, and that includes Thanksgiving (which is, I know, still a few weeks away). You should prepare for this special day physically, emotionally and spiritually.
First, prepare physically; that is, do as much as you can in advance. You know better than I what this might involve, but the goal is to avoid being overwhelmed at the last minute. Make lists of what you'll need to do and when. If you can bake or freeze anything in advance, do it. And don't feel like you have to handle everything; get some of your relatives to help you, if they don't already.
Then prepare emotionally. You know the stresses you'll probably face on the holiday; set aside time to be alone and find moments of calm. Make prayer a part of your preparation also, asking God to keep you from getting upset if things don't go quite the way you planned. In addition, take time to reconnect with those you don't see regularly; some of them may not be around next year.
Finally, prepare spiritually by making these weeks leading up to the holiday a time of true thanksgiving. God has given us so much, and we can be so ungrateful. Above all, give thanks for God's greatest gift -- the gift of His Son. The Bible says, "Give thanks in all circumstances; for this is God's will for you in Christ Jesus" (1 Thessalonians 5:18).