This is good English! Some formulations sound a little unnatural. I would add an exclamation mark after 'Shiori' but it's really up to you! For the sentence 'I want to visit your country', if you have already been and want to go back you should say "I want to revisit" "return to" or "I want to come back to your country". Hello,* this is Shiori. Thank you for all your support during my stay. It was a memorable experience. It* was my first time going abroad,* so everything was new and exciting. I'm sure that I bothered you several times. I asked you to repeat *yourself [or something/things] so often and I couldn′t make myself understood in English, but you were so kind to me every time. I can′t thank you enough. I have been practicing *hard with my English since I *came back to Japan, it is still bad though. I treasure the time I spent with you. I want to visit your country and I hope to meet *you again someday. Thank you for everything. Shiori.

Note: Next time, put this in the NOTEBOOK section as it is easier to edit there, and you will see the edits/corrections better. Hello. Thank you for all your help during my stay. It was a memorable experience. This was my first time abroad, so everything was new and exciting. I'm sure that I bothered you several times by asking you so many times to repeat something and when I couldn′t make myself understood in English, but you were so kind to me every time. I can′t thank you enough. I have been studying English diligently since I came back to Japan, though my English is still bad. I treasure the time I spent with you. I hope to visit your country and meet again someday. Thank you for everything. Shiori.

Dear Fitzgeralds, <-- use a comma Hello, This is Shori. Thank you for all your support during my stay! <--- It was a memorable experience. This was my first time going abroad, so everything was new and exciting. I'm sure that I bothered you several times. I asked you so many times to repeat *something* and I couldn′t make myself understood in English, but you were so kind to me every time. I can′t thank you enough. I have studied English since I went back to Japan, *still* my English is still bad though. I treasure the time I spent with you. I want to visit your country and I hope to meet again someday. Thank you for everything! Shiori <-- no period after your name

↑It helps me a lot^^;

Thank you! It helps a lot!