This may not be what you want to hear, but I got a feeling this revolves around emotional maturity. It could very well be that neither of you are adequately equipped to deal with the convoluted emotions which being best friends, dating, then going back to best friends can bring. It. Is. Hard. This ain't to say you are deficient or inadequate...quite the opposite. Unfortunately, we only mature emotionally through experience, hard knocks, spade work, life struggles.
There has to be balance. If you are equipped to deal with the issue but he isn't, you will have imbalance (things just won't feel right). Likewise, if he is equipped, but you aren't. And if both of you are not equipped...only time will bring resolution.
Now, I've dated friends, even good and best friends, and they don't always work out. Yes, there is hurt. Yes, there is pain. Yes, there are mixed feelings and confusion. But it is possible to rise above that, to find the commonalities which fostered the friendship to begin with. Keep in mind, we can never go backwards, things will never 'be the same', for things change, people change, situations change, the water passes the bridge. Best we can do is to make do with what we have Now, and what might be Tomorrow.
It's a funny old world...break a bone, 8 weeks later, you're fine. Break a heart, and you may never heal...at least completely.
“It is not the strongest of the species that survive, nor the most
intelligent, but the one most responsive to change.” Charles Darwin