You're doing good things. On the topic of what is and is not "emotional abuse," it's smartest to avoid labels that are also legal and medical terms of art. Better just to state the facts of what you did that now causes you regret.
My taken on this is that the letter is not going to fix anything by itself, but if you still have any hope of fixing the marriage, and you really believe you did your wife wrong, you're going to have to acknowledge that as part of the process anyway. There's no reason not to put it into a letter.
Also, I understand why you're thinking more of the right thing to do than the best strategy in a divorce, but the rest of us are not emotionally involved in your situation and, without speaking for anyone else, I admit I'm concerned that what you write can be used against you later. That's why I suggest very straightforward descriptions of what you actually did and a minimum of editorializing about how evil or abusive you now feel it was. If things progress to the point of divorce, your wife's lawyer will not take into account that you used a label like "emotional abuse" only to describe your own guilty feelings about something you did.
