TryingToSave: I gave you the steel toed boot because your very question sounded like that was what you were asking for. I took the time to offer an honest opinion in a supportive kind of way. You have the right to take it or leave it.
Though, I have 20 years of long hard experience dealing with a potentially suicidal person and certifiably crazy person. In fact, I have committed my own mother 5 times. Thus, I speak from experience when I tell you that the worst thing you can possibly do is use the fact that she is an emotional mess as an excuse. Life has consequences no matter how happy or sad you are. “Being suicidal” does not give her carte blanch to ignore the basic rights of another human being.
Let me break this down to another level for you. If she really is an “emotional mess”, then the very fact that she came running back to you undeniably proves that you are her safety net. Furthermore, your W confessed her indiscretions to you in detail because she essentially thinks that will gain her forgiveness and absolve the offense. Though, don’t you see that by selfishly pursuing her absolution that she completed ignored how that would make you feel? You acted “as-if” because you were secretly stifling all the pain that the event was causing you. More over, you were afraid to stand up to her and say, “No, hearing those details is not going to do any good. In fact, it would hurt me and further drive a wedge between us”. You apparently skipped that chapter in the DB book. You let go of your own well being out of fear that if she knew the truth that she was leave or hurt herself. I understand why you did it. Though, it is the wrong way to handle the infidelity issue.
There is nothing wrong with being the rock. If fact, I am flat out telling you that you actually need to be the rock! Though, being “the rock” means more than just offering unconditional love and support. It also means that you need to enforce the basic rule of life and human existence too. I am not saying that you have to ride the high horse of pride and punish her. Though, I am saying that you owe it to both her and yourself to clearly define that her actions where inappropriate and hurtful Most importantly, she needs to know that only true work over time will actually absolve her and regain your trust. Caving like a marshmallow, not enforcing consequences, and letting her live in completely self-centered world will not help either of you.
Good luck.
By all means marry; if you get a good wife, you'll be happy. If you get a bad one, you'll become a philosopher. -- Socrates