Present perfect or past simple

I'd like to clarify the question concerning the use of past simple and present perfect in the following sentences:

• I'm sure you were said this phrase by others at least once in your life, especially when you experienced stressful situations

• I'm sure you have been said this phrase by others at least once in your life, especially when you experienced stressful situations

• I'm sure you have been said this phrase by others at least once in your life, especially when you experienced stressful situations

I'm not sure about the use of present perfect with the phrase once in your life, but then I have the second part of the sentence, in which the word "when" can't be used with present perfect in most of the cases (as far as I know).

• As I (have) mentioned before, I'm living in a small town 

What tense should I use there if I want to emphasize that I (have) mentioned this fact previously in my essay/conversation?

• It turns out that I (have) left my rucksack in the corridor.

Can I use present perfect instead of past simple, even though that this even occurred a long time ago?

• In your last (recent) letter you have written (wrote) me about your studies

• Thanks for your last (recent) letter in which you have written (wrote) me about your studies 

In this sentence I have the same questions as in the previous ones.

Can you tell me which variants of these sentences are correct? Sorry that I have asked so many questions in one topic.

Thanks in advance!