I've got a sackcloth and ashes you can borrow if you wish. You've apologised. I think she knows you're sorry but she isn't interested. Don't do that awful thing where you keep saying over and over, louder and louder, slower and slower because you haven't got a response from her.
I too am stubborn but I can promise you that she is going over and over in her head everything that you've said to her in relation to this. Also, when we're angry we rewrite history and everything that you ever did was wrong, immoral, insensitive etc etc It fades with time. Give her time and space.
However, what I really want to ask you is why are you facilitating the estate agents? Why have you responded to her requests to do so when she is on holiday. Let her do the hard work. Don't stand in her way but don't do it for her. You see in our sometimes complex female logic, we can turn round your nice guy actions in our heads and come to the conclusion that you don't want to be married either. Otherwise why would you be helping. If you pursue and are all sorry all the time we see you as weak. If you are nasty then it shows us that we were right about you all along.
The point is that you are very unlikely to get it right or choose the right option. It may seem unfair but because she has decided that she's done, whatever you do can be misconstrued.
The guys on here are often heard to say that you can't nice her back. You also can't force your remorse on her. It is her right to not accept it and to end the marriage if she wishes. At the moment she wishes to, so have some respect for that. It may turn out to be all hot headed celtic blood.
In the meanwhile do all the DB basics if only to stop yourself obsessing and definitely stop yourself from sending a letter.
She only needs to glance back once and see a strong, together man who obviously has remorse for it to sow a seed of doubt in her mind about what she wants in the future.
Do not panic. Gain yourself some time wherever you can.
