I was in a similar place with my first long term bf. We did every thing together, from work to travel. We lived together and had a good relationship. Later, Travel kept me away from home and we discussed it. Though we loved each other and lived together for another 2 yrs we broke up as a couple.
He was still my bf and I was very much in love but understood. He starting dating a girl and it wasn't cool in my jealous mind somehow I figured it made more sense to keep him in my life then end our friendship.
He asked me to be his best man, it twisted me up in a knot just the fact that he was getting married but to be the best man for a guy I had been with and living with for 9 yrs since 18!!! I loved him very much but well what can you say?
Sure, was my answer to being the best man.

It broke my heart but made me happy that I counted that much to him and many friends thought I was a fool and how could I do that as we really were such a couple doing almost every thing together socially plus shared the same taste in clothing to what we liked (except I am gay). I have no regrets but it was fucking tough. We talked about this and it got a little emotional between both of us more than once with watery eyes but it was how it was to be for him regardless if I agreed or not and what friend denies another their goals when you start out young things will change?
It was good being his best man and the wedding was very nice. His wife was aware of our relationship and between her and me there was some tension. However throughout the years we remained casual and friendly until she tried to involve me in a ARMS mortgage scheme.
A couple weeks before he married he moved out. Directly after his marriage he moved to the other coast for a job. It was all positive for him. I was happy but shed more then a few tears. Luckily I met a cool guy almost the next week after the marriage and that turned into a lifetime partner but I still think of my best friend who treated me well in all aspects but had to follow his heart. I still wish he was in my life, however we drifted as things happen with all those miles and that was likely for the best.
It was and is not easy but love takes 2 to tango and just because the relationship changes does not mean that true love cannot flourish in a different form just as a tree is green and fresh in the spring, and in late fall the leaves change as they fall off but it is still a good tree that has many more years of shade to give if you allow it to grow.