Alwaysneedinggoodadvice my ex can also seem to move on from person to person with relative ease. In my case it was an affair initially, the speed in which I was discarded was frightening.
3 months after we split, the AP died suddenly (Some might call it Karma) 4 days later my ex had signed up for on-line dating! All this 4 months I left after discovering the affair!
I had counselling, it was great form me and provided me with validation that I wasn't crazy, low self esteem and confidence maybe but I slowly began to return the person I used to be.
I have a friend who is a mature student studying a masters in phycology, she explained to me that people like your ex who can move on so quickly after relationships are very insecure people, simply can't be alone. She even suggested that they may have some sort of personality disorder, and whilst she couldn't give a full or professional diagnosis she did suggest there were some pretty strong narcissistic traits with my ex's behaviour. I had heard of narcissism but had no idea how it could be hidden in a persons personality and me not recognise this for nearly 25 years.
It's been 3 years now since my marriage ended and I've read loads on personality disorders and it's on now that I've realised that although my ex may not have full blown NPD they do have some pretty strong traits, and a shitty character, lack empathy and a severely wonky moral compass.
I miss who he used to me. He used to be my best friend. We used to have an amazing marriage
I felt like this too, but on reflection I realised that my ex was covertly emotionally abusive. We met young and everything happened quickly and I fell in love hard. This is called the 'love-bombing' stage. Where they make you feel like you have met your soul mate, they seem perfect.
Then slowly things started to change, very subtle at first, but the red flags were there I chose to ignore them. I felt like i was being used, and bullied. Not bullied in a physical way but more emotionally, I felt stupid and ashamed about this and never told anyone out of fear that I would seem crazy. this stage is called the 'Devaluation' stage in the relationship.
Finally there was the affair, and the speed in which our marriage was thrown away, this is the 'discard'. You're tossed away like a piece of rubbish and they move on with their lives with apparent ease.
Maybe your marriage was different, but I know that I felt the same as you in that i felt we had a decent marriage, one that wasn't as unhappy as my ex made it out to be. If our marriage was unhappy then I'm afraid there must be a lot of unhappy marriages out there.
Take time to heal from the shock of your relationship ending and the speed in which your ex moves on with a seemingly "Happy" life. Like others have written see this as an opportunity to get the best revenge on your ex as you can...live a full and happy life. Find a hobby, get fit, join a gym, join a club...do something you've always wanted to do.
My advice would be to see a solicitor and start divorce proceedings.
Do you have kids? If so limit contact with your ex, don't give him any attention other than formal arrangements about the kids welfare and access.
If not block him on social media and by phone. Go 'no contact' speak to a solicitor and tell your ex to direct anything via your solicitor from now on.
I'm so sorry you are going through this. It's a surreal and horrible feeling right now, but you will get through it, find your rage and direct it to getting through this and get on and live a full and happy life. Good luck!