The first sentence is fine except that you need to capitalize Christmas. The first part of the second sentence if fine, but the rest needs work. Try:Thank you so much for inviting me to spend christmas with you. It was very kind of you to invite me, and how much I wish I could accept it.
- It was very kind of you to invite me, and I wish I could accept your invitation.
The word it in the original refers to a word that isn't in the sentence. You could also say:
- It was very kind of you to invite me, but I must decline the invitation.
That might make sense over the telephone, but I wouldn't put it in a note. In any case, you can't use not, because it contradicts impossible. (You could say not possible.) Another problem with that sentence is that it unnecessarily repeats what has already been said. Perhaps:However, it is not impossible for me to come over to join you and I will tell you why later ( or in a while?) because mum's rushing me into going to the supermarket now.
- I will explain later.
- I will give you the reason when I see you in person.
What do you think?