It happened to me around a year ago. There was no other woman in the end, he just decided he didn't love me anymore and he'd obviously been pretending nothing was wrong while making the decision to just leave. In a way that was harder to understand.
A few reflections which might help you.
The first is that you will naturally look to yourself and wonder what you did wrong, what is wrong with you and all those questions. The answer is that it's not your fault. Not even 1% your fault. If someone is in a long term committed relationship then they have a responsibility to the other person to communicate to them changes in how they feel or what they want. If they can't do that and decide to keep quiet and suddenly leave it's a deficiency in THEM not YOU which causes that behavior. Sometimes a person just falls out of love - but the essence of commitment means they have an obligation to at least talk that through and make some sort of effort to work on it. Even if it doesn't work, at least the other person a) has time to mentally adjust to what is happening, b) gets some understanding as to why it has happened and c) feels like at the end of the day they were worth bothering with and earned the right to be treated like a human being.
So much of your self worth will be tied up in your belief that he loved you, and when that goes and he treats you like nothing it has a massive effect on the self esteem. I think though that when someone treats you like shit it says more about them than it does about you.
When this happens, people around you (friends, family) will act strange towards you. They will be as shocked as you are and they will not believe your story that he "just left" because they think there must be some explanation for it and they will gossip and judge and act like you had a bad relationship. This hurts a lot......try and remember they are just as blindsided as you are and ignore it. The truth will out eventually.
This is one of the most painful, horrendous things anyone can go through. I am just at the point nearly a year later where I don't wish I was dead almost every day and I am quite a strong person I think so to have been that low for so long was suprising. I just never in my wildest dreams ever believed he would behave like that to anyone - much less me, who I thought he adored. It's just traumatic and the grief comes in so many layers, in so many diferrent waves and you feel like you can't survive it but you can and you will.
Slowly you'll start adjusting to life without him, and slowly you'll start detaching emotionally from your planned future with him, and slowly you'll find a place in yourself where you think; "hmm...well maybe i want to be with someone who's committed to me, who behaves better when hard times come and who loves me just as much back".
I am not there yet, but you will feel yourself growing stronger each month and you just have to keep the faith in that. No one's destiny is tied to someone who leaves them. I read that somewhere and I believe that is true.