'I accept everything that has happened did so because you were unhappy in our marriage.

Do you trully accept that? Are you being honest here or you just want her to see your changes?

I didn't realise things then that I really should have and obviously you had been unhappy for a long time.

Life its a learning experience, are you working on those issues?

All I can do is apologise for not doing enough to look after your needs in the way I should have.
I realise now that there were so many things I paid no attention to when I should have. I can't change what has happened in the past and that saddens me.

Why would that sadden you? How could you be a better you if you didnt experience all this?

All I can do is continue to be the best Dad I can be for our boys and I hope you find happiness in your life'

I dont think you should say so many words, more actions will be better.

Does that sound too weak or forgiving?
How do you think it sounds? If you bring up those 2 words, maybe its because you identify them in that text...

Any advice on what I might or should say would be appreciated.

What has been done its past, if you are gentle with yourself you will be able to accept that, everybody does "mistakes" including her...

What about:
I understand how you felt in the M, and I am sorry you felt that way.
I apologize for the issues that arise and how I contributed to.
Let me know how can I help you with all this situation and if there is something that I could do for you.

When the student its ready, the teacher will appear...
Even after all this time the sun never says to the Earth, "You owe me."
Look what happens with a love like that,It lights the whole sky.