Scarlett, I am also in the same situation.
Me and my best friend have been close for many years, we had a bit of a think about 5 years ago, and one min he wanted a relationship with me and I didnt, when I thought I wanted a relationship, he didnt. But it never worked more than just friends, and to be honest I just dont fancy him. For the past 4-5 years we have mostly been fine, but when either one of us gets a new partner, the friendship does go a bit weird.
I have been with my current partner for 2 years now, and my best mate, has sometimes been a bit inappropriate, and I told him he had to deal with it. Now recently he has got a new girlfriend and I was jealous and it is weird, but I dont want to be with my best mate, not at all I am very happy with my partner and could imagine anything worse than being with my best mate.
Now he has a new partner, I barely ever hear from him, and Im a bit hurt that he has time for her and not me. I think a lot of it is to do with you not being the main person in his life, and he is putting other people first. I suppose you could apply it to if say my mum got a new partner, I would get jealous of my mum giving someone else attention.
That is my experience anyway!!