Ok this is never an easy story to tell...

About 3 weeks ago, I broke up with my fiancé and left him for one of his friends.

We were together for about 2 years. I was always the dominant one and he hated it, thus resulting in constant fights. We loved each other dearly and eventually got engaged (regrettably mostly beacause I wanted to), but regardless of the fights and the fact that we kept on breaking the engagement off, things were great and the best 2 years of my life by far!

In about the last 3 months of our relationship, we started on plans for The Big Day and amongst all the excitement, I started realising that I was doing most of the arrangements myself and that I was getting little (if any) help from his side.

Now, me and his friend have always been good buddies, we practically saw each other as brother and sister. In the last couple of months I spoke to him a few times about the wedding plans that I was always doing myself and the fact that the fighting between me and my fiancé was increasing, the more we tried to work on our relationship.
The friend then eventually came out and admitted his feelings for me...
I knew things between me and my fiancé weren't working out and I couldn't see things going further, the spark was gone.
At about the same time that I broke things off with my fiancé, his friend broke up with his girlfriend too.

It's been 3 dreadful weeks of my ex's family and his freind's family hating me (I don't blame them) and it's been really difficult. My ex won't leave me alone as he 'believes' I was put on this earth for him and him alone. Now he wants to be friends but almost every time we text, he has a breakdown. He keeps talking about what we had and how good it was.

I know what I did was wrong... I mean, I broke up to relationships simultaneously... I guess I just don't know what to do anymore.
How do I help my ex move on so that I can move on too? He says It'll kill him if we break all contact.

Is there any help out there?

You break all contact with him so he can heal. That is the only way he will be able to get over you. Since you were the dominate one in the relationship, just tell him that for his own good there will be no more contact and then you, for his sake, stick to it.

Ok but how do I tell him that I'm going out with his best friend? He already has his suspicions but to confirm them will be devastating for my ex.

That's not his business anymore, that's your business. Let it go.

You don't have to tell him that. Just leave him alone and go on with you life.