Letter Ffor a Friend

Less important words are typically not capitalized in titles, and an article is typically needed before friend.

Hi, darling Elizabeth!

The comma can be added by for stylistic reasons but it not needed. Using darling is a very friendly and almost romantic greeting unless it is towards your child.

I hope that you feel yourself better today.

I want to give you the prize as the most principle human so inside my letter you waited a sweetness -some advice since you are my close friend so inside my letter is some advice: don’t think about your neighbor’s words.

This sentence is very confusing. I think my correction still what you wanted. "principle human" does not make a lot of sense in this context. I think you mean a human (person) who you are friends with, so I changed it to friend. I replace the "-" with a ":" to show that "don't think about [your] neighbor's words" is the "price" (advice) you wish to give your friend.

Please let me know if I am wrong.

I guess she was upset that you didn’t invite her ato the club and that is the reason why she said you not nicemean things.

Not nice is technically correct, but rarely used.

Today I am feel myselfing a bit secretly -ive - I don’t want to ask you several questions about my plan, but haven’t got aecause I will not have any regrets!

"feel myself" is not a common phrase, instead you can say "I am feeling"
I assume Elizbeth already knows about your plan. If not this sentence should explain what the plan is.
As @Deimophobe mentioned, you cannot regret future actions, so adding "I will" means you are confident after the action takes place you will not have any regrets.

I want to change one course in my uni because I don’t like the professor and the lesson iss are boring.

My question for you: what is the most valuable thing in life forto you?

For is okay, but "to you" is more natural.

(With the b) Best regards, ¶
Co

"With the" is very extra and stylist, most people in the US just say "Best Regards".

Letter Ffor a Friend

I'd usually keep 'for' as lowercase in a title (unless it's the first word), but this is not a hard rule.

Hi, darling Elizabeth!

This sounds a little strange - I'd go with something like "Hi/Hello/Hey X", or you can do "Dear X" if you want something more sincere or formal.

I hope that you feel yourself better today.

I want to giveaward you the prize asfor the most principle humanimportant person, so inside my letter you awaiteds a sweetnesstreat - don’t thinkworry about your neighbor’s words.

Is the the 'sweetness' meant to be the advice about the neighbor's words? If so, you may want to use the words 'some advice' instead of 'a treat'.

I guess she was upset that you didn’t invite her ato the club and that is the reason why she said you not nice thingsmean things to you.

Today I feel myself a bit secretly -ive - I don’t want to ask severalyou to questions about my plan, but haven’t got aecause (I know) I won't have any regrets!

You can't regret things that haven't happened yet, so you have to say you won't have regrets in the future. You can add "I know" if you want to sound more assertive.

I want to change one of my course in myat uni because I don’t like the professor and the lesson is boring.

My question for you: what is the most valuable thing in life forto you?

"to" sounds a little more natural, but "for" is fine too.

I hope that you feel yourself better today.

I want to give you the prize as the most principle human so inside my letter you waited a sweetness - don’t think about your neighbor’s words.

I guess she was upset that you didn’t invite her ato the club and that is the reason why she said you not nice things to you.

Today I feel myself a bit secretly - don’t want to ask several questions about my plan, but I haven’t got a regret!

I want to change one course in my uni because I don’t like the professor and the lesson is boring.