I am glad to receive your letter.
As for the problem mentioned in your letter, I think it's a good idea for you to talk to your mother.
In your conversation, you should first tell your mother that you are very grateful for her concern. Then, you can tell her that you are not a baby who always needs a lot of care and worry. You are old enough to take care of yourself. But remember, the only reason your mother did this was because she loved you so much.
I hope you can talk to your mother.
salute
yours respectfully/sincerely
Li mei