It is ok to be scared. It's ok to feel crazy. You are thinking about undergoing your OWN stages of grief
and mourning the loss of the relationship if you break up and it frightens you. Who loves signing up for grief? Nobody.
But first... before you can grieve, you have to actually break up. Deciding to do that is part is the roller coaster of the cycle of emotional change.
You seem to be at "self doubt." You are getting close to "acceptance" and accepting that a break up is due here. Just not quite
It's ok. Feel whatever you have to feel and let it blow on through.
It's scary to think maybe he doesn't want to be friends if you break up. I get that. You didn't think you would break up so you never thought to ask him ahead of time. But you know you. You aren't going anywhere. You are still there in his life as a friend if he wants to be friends.
You have emotional responsibility over your own self. As does he. YOU don't "make" him feel whatever. If you had that power you could wave a wand and POOF! You could make him feel happy.
What you control and can change is your
I see that you don't like a loved one hurting. But again... pick what hurts least. If there were a "hurt free" option I'm sure you would be on it like white on rice... but there isn't one of those options around here. So could move it forward.
Let yourself be ok feeling the emotional cycle of change feelings as you think about making this big change. Feelings won't kill you. Accept it is going to feel all roller coaster. Then when you break it off, accept you are going to feel the new
roller coaster of "stages of grief." Both of you.
Then hopefully in time, you will both feel better. And you will both still be friends, because you share companionate love. Exercise some faith in yourself and in your partner.
In your next relationships, you could talk about how to break up ahead of time.
Then if it should come to be needed? That emergency plan for a good break up? It minimizes some of your discomfort in the break up time. Then you could know AHEAD OF TIME that your partner wants to be "good exes and friends" afterward and not have that unknown amplifying the "scary."
I think part of your ugh here is fear that he might NOT want to be good exes and friends. So you fear a double loss -- the loss of the romantic relationship and the loss of his friendship. You never thought to ask what kind of ex he wanted to be because you never thought he'd be an ex.
But being afraid to leave because you might not be friends later is not a good reason to stay in something that you are not really into any more. It's also not very "friendly" to keep things dragging on. You have to stop being a GF. But you can continue being a good friend here -- man up and do what needs doing.
You haven't done anything wrong or made any mistakes. You grew. So did he. This shape no longer fits. That is all.
Hang in there.