I saw your post yesterday and I went away hoping that someone, with more wisdom that I would post here first, because I wasn't sure what to say that would help.
However, you are going through a lot of pain and I do know about heart-ache and broken relationships.
It is obvious that you are in shock, even if you had an inkling that your partner might be going back to this group, you were not expecting to be cut out of his life like this.
You know what it sounds like? It sounds like a bereavement - like when a spouse dies, that sort of a shock. I am told the pain can be almost unbearable.
I would say that there is no easy fix for this,
Just like someone who has had a bereavement, please find a non-judgemental person to speak to and pour out your heart. I believe that you will have to do this quite a lot.
I think, in my view, managing the pain and looking after yourself - eating as well as possible, sleeping as well as you can- will help initially.
There is a text message service run by sane, a telephone talking service run by sane and there are the Samaritans, they will be available so you can talk.
These things are a bit like first aid, in later weeks you will be able to deal with the other stuff, the exclusion and rejections, and you will be able to slowly come to terms with what has happened.
I hope this is a bit of help.
I am sorry that this has happened.
P.s. just an idea -never think that you were/are not good enough for your partner, there are things in other people's lives that we can not control.