How has my ex got over our relationship so quickly?

We were in a relationship for 13 years, even engaged for the last year. It was a good relationship. We tried to live together and obviously jumped in feet first without talking about what we both wanted from each other. It got difficult for me as I have lived alone for 12 years and found it hard to live with someone else. I went back to my flat. After this our relationship went downhill and he broke it off with me.
We have had other stuff happening in our lives that hasn't helped. Such as death of my mother and his dad and also I have a teenager from a previous marriage who was playing up and testing my sanity.
we stayed friends and always keeping in touch. I found out in April that he was dating someone else. Not from him but from the girl adding me on FB and then putting photos of them together at restaurants. I asked him and he said its true but he has been trying to break up with her but she has tried emotional blackmail and asked him to allow her to break up with him ( this hasn't happened yet) he told me she is jealous of me and asked him to stop talking to me, which he said no to.
They started dating straight after we broke up. How did he jump from one relationship to another? Did he never love me? Is he over me so quickly?
i have decided to stop all contact with him so I can get over him, I would run back to him if he asked. Hi am confused as he always texted to cj
heck up on me, he texted me happy valentines when he was still with her (at the time I didn't know) why is he sending me mixed signals? Did he want me as a back up while he dated her?
I don't understand what he has been doing.
Now I have blocked him on whatsapp and I don't have social media any more as his girlfriend has been stalking me all all of them.
Please help me! i am still madly in love with him.
How has my ex got over our relationship so quickly?
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