Formal thank-you letter

[Kitteeh]

Me again...
I have to write formal thank-you letter for my imaginary English teacher.
Could you check this, please? Advices are very welcome. I would be grateful, as always..

Dear Mr Surname,

I would like to thank you for all the things you have done to help me with preparing for the Final Exam. I am especially grateful for your patience and kindness. I must mention, that the time I started preparing for the exam, I was completely unconfident () about my skills and I even thought I had a very few chances to pass the exam. But you managed to build my confidence and knowledge. Also, I am very grateful for your advices about relaxing and keeping myself calm at the exams - it really worked.
So, the results of our cooperation surpassed my highest expectations.
Thank you once again for working with me.

Yours sincerely,
Name Surname

[Ola2705]

Dear Mr Surname,

I would like to thank you for all the things you have done to help me with preparing for the Final Exam. I am especially grateful for your patience and kindness. I must mention, that the time I started preparing for the exam, I was completely unconfident about my skills and I even thought I had a very few chances to pass the exam. But you managed to build up my confidence and knowledge. Also, I am very grateful for your advices about relaxing and keeping myself calm at the exams - it really worked.
So, the results of our cooperation surpassed my highest expectations.
Thank you once again for working with me.

I indicated the mistakes in red.
1. You cannot put an indefinite article before plural
2. Advice is an uncountable noun. When talking about more than one advice, it's better to say a piece of advice