It's Not Over 'Till It's Over

[planet]

Before we were married she promised me that she would never leave "no matter what" & then we married and vowed "for better or worse, till death us do part".

She is the only person I have ever trusted with all my heart & that has just been totally destroyed.

I feel like I want to know the reasons because I don't know how else to accept that someone who I trusted & who made promises to me has gone back on those promises. I want to know the reasons to explain or understand how those promises can be broken.

I understand this. I got the same promise too.
Understand that your W did not arrive at her current decision in a day or two. It probably took her months or years to do so.
Her love tank is pretty much used up.

I think I have figured out some of the things I have done that have contributed to the problem and have already begun working on improving those areas.

Good stuff.
Recognizing your faults and owning them is the right way.
I guessed you have hit rock bottom. This is when things become very clear to you regarding your actions and/or inaction in your M.

I do wish my wife would believe that though.

Do it for you. Not for anybody else.