I only want to answer this question from my own experience. I met my best friend just over 2 decades ago. We grew up together, knew each other's families and a lot of the same people. We stood by each other's sides through everything - there's nothing we have not shared or experienced together some way, somehow.
A few years ago, we took it a step further and dated. We thought nothing would change, but I soon realized that we were wanting two different things. We failed to realize that being in a relationship will inevitably change things, as you are no longer "just friends". It was too much for both of us.
Let me admit, I was the one who walked out. While I almost felt rejected (because it always felt more like a friendship rather than the progression of a relationship), it was actually him that felt the rejection. How ironic. This led to hurting on both our ends and we had an ending argument in which we didn't talk for quite some time.
Over time, I wanted to reach out to him because I missed our life long friendship, but my pride (and fear) would not allow me to. I finally did so after 2 1/2 years. Apparently he was trying to reach out to me as well - thank God :)
We rekindled our friendship and it's almost as if none of it ever happened. Why were we able to do that? Because this is a true friendship. We are meant to be friends, not partners. We have a love for each other that no one in this world can ever duplicate or replace - there's no "we" like ours.
You have to have a friendship in order to have a successful relationship. But friends aren't always the right ones to create a relationship with.
Our story is unique as far as we were able to renew our friendship. Not everyone will survive that. But at the same time, there are many people who marry thier best friend and stay that way. It's very individual.
My suggestion is to be sure this is something the two of you actually want to try to do and make an agreement that if either of you feel like this isn't the right thing, you'll be honest about it, and promise not to do anything that will hurt the other one on your way out, promise to never let your friendship die if the relationsihp doesn't work. If you need some time apart to heal, fine - but not forever. Forgive and forget and move forward. And if it does work out, then congratulations - you're two in few! :)
God bless you both...