But you should and need to tell them soon.
Can i suggest you throw a small but special get together first? Or have a normal but very memorable get together that is a little special but not concerning. give them that last big moment of bliss beforehand, and make it your own goodbye party without them knowing. You can have a wonderful time with everyone, eat your favorite things, favorite music, favorite movies, favorite people.
Then tell them the day or two after.
Dont ever worry that they will resent you after you leave this world, especially after the time you have given them. You gave them such an increbibly selfless gift. You gave them the one thing everyone wishes they had with those they love...time.
They will always love you.