Making an apology: Polish grammar construction

Hi/Hej
Please, what is the equivalent Polish construction for 'wishing to apologise for not having done something' - I get stuck every time! So annoying! Your help in resolving this probem will/would be greatly appreciated. Dziękuję bardzo...

Example-Przykład:
Droga Aneto
I apologise for not having replied before now, but...
In Polish = ...

Przepraszam, że wczesniej nie odpisalem, ale ...

You could also say,
Droga Aneto,
Bardzo mi przykro, że nie odpisałem wcześniej ale...

Michal might be right but I thought mi przykro was used in sense like 'I'm sorry that your friend died; I'm sorry you are not feeling well. I thought mi przykro has more of a sense of 'being upset' rather than the sense of 'an apology'. But I'm also a beginner in Polish so may well be mistaken. Comments by more PF members would be appreciated. Dziękuję bardzo...

Don't worry, Michal is a beginner too.


You're right.
Logically:
Przepraszam - for something you had an influence on. (you chose the moment to reply to the letter, so it's your fault, you might have been really busy etc., but you set your priorities, and replying to someone's letter wasn't at the top of the list)

Przykro mi - for something beyond your influence.
"Przykro mi, ale to co mówisz jest bez sensu" - "I'm sorry, but what you're saying makes no sense." - Here you're making an unpleasant comment, so to attenuate it a little you can add "przykro mi", but you don't have to apologise for it, because it's the other person's fault that his/her words make no sense.

Many thanks Krzysztof. Your help is really helpful and easy to understand. It always makes sense! Dziękuję bardzo...