There are two weaknesses to an impersonal style in letters.
1) It's unclear.
2) It's inhuman.
Get the "you" attitude. Don't think you need to fill your letters with I, my, we, and so on. If you think only of yourself, you invite resentment. Visualize your reader. Picture the signer of the letter talking face to face with the reader. Then write as the signer would talk - not exactly, but like it.