Job invitation

A few months ago I had a English text in which I needed write a essay with this title. I would like if someone could revise it for me.

Dear Mr Antoni, 
First of all I would like leave my sincere gratitude in receipt of this good proposal. I am very proud to know the respect that you have for me.
Despire of I really wanted to get this job opportunity. when I know it will be so important in developing my career in our company and it could also be a great chance to improve my English language. At the same time, I could live in that amazing country with such a different culture, language and lifestyle.
How could I write this message without leaving bad feelings about my ability to hold this position? Unfortunately at this delicate moment in my life when I need take this difficult decision in my professional career, when something very bad has been happened with my family and because of this I can't able to accept this great opportunity. It is more necessary I remain here together with my family.
I am extremely sorry about this, but I hope you understand my situation and not think that it is not a simple excuse that I made. Sincerely I want to receive this proposal again in the future, when I can enjoy it with great enthusiasm.