I am a 32 year old American male. I dated a Japanese female from Kobe for three years. She is now 35 years old, married with a baby. I met her in Kobe, where I lived as well. We haven't spoken in about four years.
This summer I am going to Kobe and expect I will see her. The reason is because I would like to apologize for how I treated her. I will write her a Facebook later asking if we can meet, whereupon I will explain myself.
My question is what kind of language should I use? How formally should I write the letter and speak to her in person? Should I say "お久しぶりです" or "久しぶりやな！", for example. Or 神戸弁で、「最近何しとん？」? (When we were going out and even just friends afterwards we would always use the casual form, and often Kobe-ben 神戸弁. This was in a kind of playful way, because she is from Kobe, and I love the Kobe region.)
Since the purpose of my meeting her is to apologize for some of the things I did, there will be a tone of remorse. However, she is a very warm and kind person, not too formal either, so I expect she will still treat me kindly. Thus I do not want to be too formal if she will not be. However, I do not want to inappropriately suggest intimacy that is no longer there, if doing so crosses boundaries.