*snugs a little tighter*
That sounds very rough
Sometimes the feeling of slipping away from one another can be worst than an angry fallout. There's a lot of unknowns that come from the gradual separations, a lot of what ifs.
But really, when it comes down to it, its not so much about what happened to cause it so much as it is life gently telling you its just not the right person. Whether that be friends or dating.
Growing up and going off to college and other stuff have lead to a few of these myself. And though they are always rough, it always gets so much better after the storm. The others who step into the space, and fit correctly, make the hard moments like these worthwhile.
Btw, totally not saying you can't mend this or anything, just saying if it does fall apart, its going to be okay.