I actually have been in similar situations.
Like not my bestie but a really good friend proposing and also actually pushing for it, but I had to take a stand and just keep the facts upfront that they are a great human and I see value in friendship with them , but I don't feel like wanting to pursue a relationship with them for sure..
Everytime I have been asked for reason, i could only come up with that I genuinely never felt romantic for you. And it's as simple as that.....never felt romantic, never came the thought of picturing a life with you as a partner...not before them proposing and neither after that so it absolutely definitely is a no from my end . (Ofcourse you got to put this in as gentle and polite way possible, because we don't want to hurt them )....but that's the the truth.
And as you are afraid of, I lost the friendships....but here's another perspective.
I have been friends with this girl for over a year and a half and I am the closest person to her right now. She has been having a really really really bad time since I met her but despite that she has always been strong and always shown up to fight all her battles and win over them. I have seen her in the worst of her times and despite that I have always been found of her and I see want to pursue something more with this friend π.
Now , I have been sure of this since May and I since I have been in the reverse situation in the past.....I totally understand that they don't see me romantically and that she might not see me the way I see them and that is something i would be completely completely okay with.... because I have been on the other sides too.
Regardless of not being afraid to propose to her since May, I have not yet and plan to do it in December instead. Reason being that she needs a friend more than a partner right now and me bringing in a potential drama in her out of no where would not be the best thing to do as a true friend. So I will wait till December (as we expect clouds to clear up by then).
But this is a very very dicey situation for me. Like I don't want this question mark to be on me for infinitely long. A yes would be great, a no would be fine to as I will atleast reach a conclusion than be in the chaos i have in my mind right now.
I don't know if it's a no, I will be hurt and so will she be (because we don't know what will happen to our friendship).....but i thinking it will just end in a bummer.
I hope she will understand that it's not that I do not respect our friendship and that I am willing to toss it up for no reason, but the fact that it's incredibly difficult to see someone you like and care for infront of you but not be the person in her life who actually has the right to do this completely is painful.
This would potentially stop me from moving on in my life to someone else as well.
So I guess once i propose..I will let things be as natural and let them flow in direction it naturally will. I will though ensure my interests too that I do not get hurt any further in the attempt of trying to bea friend when I actually might not be capable of doing that . (And I wish and assume that she will understand this too)