Help! He lacks Trust in Me.

Here is a message we received in our inbox. We will appreciate your contributions on this issue. Remember that a problem shared is a problem half solved. If you have an issue bothering you, feel free to send a mail to duchessintmagazine@yahoo.com
Dear Duchess,
I plead to be Anonymous, I met a man when I was in a long-term relationship and ended up leaving my partner for him.
We have been together for a month and in that short space of time a lot of people have spread rumours and tried to break us up.
We got through all of that but recently he snooped through my phone and found a message to a friend of mine saying that I missed my ex partner and would take him back in an instant.

I wrote it in a moment of weakness and didn’t mean it

I love my new partner so very much and only want to be with him.
Now he has left me, saying he doesn’t trust me and can never be with me, despite being in love with me.
I have practically begged him to stay with me because I love him so much and left a four-year relationship with the father of my two young daughters to be with him.

Now he doesn’t want me and I’m devastated. What can I  do?