Hi..
Ok, so this situation might seem pointless to some but here it is...
We met through an ex of mine 5 years ago, he is still best friends with said ex.

We recently became close and decided that at he stage we are both at in our lives we are not looking for a relationship.
But as things progressed we spent every free moment together, double dates with other couples, message, skype and calls all day everyday.
Yesterday he told me that he has let me in and that hasn't happened with anyone in a long time.
I then, very stupidly tell him that this can't carry on because I have feelings for him, more than I should have and he can't give me any guarantees that he would give the relationship a chance. (He is petrified that said ex will find out)

He is now distancing himself from me and I've realised that I might be losing my best friend...

An outsiders point of view would be greatly appreciated.

If he is afraid his ex will find out, then this relationship will never have a proper chance. Its poison. He was honest, so were you, so you done good to get things in the open.

Still kind of sucks though, even if it is for the best.